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HEREDITY    AND    SEX 

THE  JESUP  LECTURES 
1913 


COLUMBIA 

UNIVERSITY  PRESS 

SALES  AGENTS 

NEW  YORK: 
LEMCKE   &  BUECHNER 

30-32  WEST  2Txn  STREET 

LONDON : 

HUMPHREY  MILFORD 
AMEN  CORNER,  E.G. 

TORONTO : 
HUMPHREY  MILFORD 

25  RICHMOND  ST.,  W. 


COLUMBIA    UNIVERSITY  LECTURES 


HEREDITY  AND  SEX 


BY 


THOMAS   HUNT   MORGAN,  Pn.D, 

PROFESSOR    OF    EXPERIMENTAL    ZOOLOGY 
IX    COLUMBIA    UNIVERSITY 


SECOND   (REVISED)   EDITION 


at/. 


gork 

COLUMBIA   UNIVERSITY   PRESS 
1914 

All  rights  reserved 


COPYRIGHT,   1913, 

BY  COLUMBIA  UNIVEK8ITY  PRESS. 
Set  up  and  electrotyped.     Published  November,  1913. 


Nortoooti 

J.  S.  Gushing  Co.  —Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


INTRODUCTION 

Two  lines  of  research  have  developed  with  surpris- 
ing rapidity  in  recent  years.  Their  development  has 
been  independent,  but  at  many  stages  in  their  progress 
they  have  looked  to  each  other  for  help.  The  study 
of  the  cell  has  furnished  some  fundamental  facts 
connected  with  problems  of  heredity.  The  modern 
study  of  heredity  has  proven  itself  ta  be  an  instrument 
even  more  subtle  in  the  analysis  of  the  materials  of 
the  germ-cells  than  actual  observations  on  the  germ- 
cells  themselves. 

In  the  following  chapters  it  has  been  my  aim  to  point 
out,  wherever  possible,  the  bearing  of  cytological 
studies  on  heredity,  and  of  the  study  of  heredity  on  the 
analysis  of  the  germinal  materials. 

The  time  has  come,  I  think,  when  a  failure  to  recog- 
nize the  close  bond  between  these  two  modern  lines  of 
advance  can  no  longer  be  interpreted  as  a  wise  or 
cautious  skepticism.  It  seems  to  me  to  indicate  rather 
a  failure  to  appreciate  what  is  being  done  at  present, 
and  what  has  been  accomplished.  It  may  not  be  desir- 
able to  accept  everything  that  is  new,  but  it  is  cer- 
tainly undesirable  to  reject  what  is  new  because  of  its 
newness,  or  because  one  has  failed  to  keep  in  touch  with 
the  times.  A  hypercritical  spirit  in  science  does  not 
always  mean  greater  profundity,  nor  is  our  attitude 
toward  science  more  correct  because  we  are  unduly 


vi  INTRODUCTION 

skeptical  toward  every  advance.  Our  usefulness  will, 
in  the  long  run,  be  proven  by  whether  or  not  we  have 
been  discriminating  and  sympathetic  in  our  attitude 
toward  the  important  discoveries  of  our  time.  While 
every  one  will  probably  admit  such  generalities,  some  of 
us  may  call  those  who  accept  less  than  ourselves  con- 
servatives ;  others  of  us  who  accept  more  will  be  called 
rash  or  intemperate.  To  maintain  the  right  balance 
is  the  hardest  task  we  have  to  meet.  In  attempting  to 
bring  together,  and  to  interpret,  work  that  is  still  in  the 
making  I  cannot  hope  to  have  always  made  the  right 
choice,  but  I  may  hope  at  least  for  some  indulgence 
from  those  who  realize  the  difficulties,  and  who  think 
with  me  that  it  may  be  worth  while  to  make  the 
attempt  to  point  out  to  those  who  are  not  specialists 
what  specialists  are  thinking  about  and  doing. 

What  I  most  fear  is  that  in  thus  attempting  to  for- 
mulate some  of  the  difficult  problems  of  present-day 
interest  to  zoologists  I  may  appear  to  make  at  times 
unqualified  statements  in  a  dogmatic  spirit.  I  beg  to 
remind  the  reader  and  possible  critic  that  the  writer 
holds  all  conclusions  in  science  relative,  and  subject 
to  change,  for  change  in  science  does  not  mean  so  much 
that  what  has  gone  before  was  wrong  as  the  discovery 
of  a  better  strategic  position  than  the  one  last  held. 


HEREDITY   AND   SEX 

CHAPTER  I 

THE  EVOLUTION  OF  SEX 

ANIMALS  and  plants  living  to-day  reproduce  them- 
selves in  a  great  variety  of  ways.  With  a  modicum  of 
ingenuity  we  can  arrange  the  different  ways  in  series 
beginning  with  the  simplest  and  ending  with  the  more 
complex.  In  a  word,  we  can  construct  systems  of 
evolution,  and  we  like  to  think  that  these  systems  reveal 
to  us  something  about  the  evolutionary  process  that 
has  taken  place. 

There  can  be  no  doubt  that  our  minds  are  greatly 
impressed  by  the  construction  of  a  graded  series  of 
stages  connecting  the  simpler  with  the  complex.  It  is 
true  that  such  a  series  shows  us  how  the  simple  forms 
might  conceivably  pass  by  almost  insensible  (or  at 
least  by  overlapping)  stages  to  the  most  complicated 
forms.  This  evidence  reassures  us  that  a  process  of 
evolution  could  have  taken  place  in  the  imagined  order. 
But  our  satisfaction  is  superficial  if  we  imagine  that 
such  a  survey  gives  much  insight  either  into  the  causal 
processes  that  have  produced  the  successive  stages,  or 
into  the  interpretation  of  these  stages  after  they  have 
been  produced. 

Such  a  series  in  the  present  case  would  culminate 
in  a  process  of  sexual  reproduction  with  males  and 

i 


2  t :  .:  k*:  .-.••:  :    HEREDITY  AND  SEX 

females  as  the  actors  in  the  drama.  But  if  we  are 
asked  what  advantage,  if  any,  has  resulted  from 
the  process  of  sexual  reproduction,  carried  out  on 
the  two-sex  scheme,  we  must  confess  to  some  un- 
certainty. 

H> 

The  most  important  fact  that  we  know  about  living 
matter  is  its  inordinate  power  of  increasing  itself.  If 
all  the  fifteen  million  eggs  laid  by  the  conger  eel  were 
to  grow  up,  and  in  turn  reproduce,  in  ten  years  the 
sea  would  be  a  wriggling  mass  of  fish. 

A  single  infusorian,  produced  in  seven  days  935  de- 
scendants. One  species,  stylonichia,  produced  in  6}^ 
days  a  mass  of  protoplasm  weighing  one  kilogram. 
At  the  end  of  30  days,  at  the  same  rate,  the  number  of 
kilograms  would  be  1  followed  by  44  zeros,  or  a  mass  of 
protoplasm  a  million  times  larger  than  the  volume  of 
the  sun. 

Another  minute  organism,  hydatina,  produces  about 
30  eggs.  At  the  end  of  a  year  (65  generations),  if  all  the 
offspring  survived,  they  would  form  a  sphere  whose 
limits  would  extend  beyond  the  confines  of  the  known 
universe. 

The  omnipresent  English  sparrow  would  produce  in 
20  years,  if  none  died  except  from  old  age,  so  many  de- 
scendants that  there  would  be  one  sparrow  for  every 
square  inch  of  the  State  of  Illinois.  Even  slow- 
breeding  man  has  doubled  his  numbers  in  25  years. 
At  the  same  rate  there  would  in  1000  years  not 
be  standing  room  on  the  surface  of  the  earth  for 
his  offspring. 

I   have   not   gone  into   these  calculations  and  will 


THE  EVOLUTION  OF  SEX  3 

ot  vouch  for  them  all,  but  whether  they  are  en- 
tirely correct  or  only  partially  so,  they  give  a  rough 
idea  at  least  of  the  stupendous  power  of  growth. 

There  are  three  checks  to  this  process:  First,  the 
food  supply  is  insufficient  —  you  starve ;  second,  ani- 
mals eat  each  other  —  you  feed  ;  third,  substances  are 
produced  by  the  activity  of  the  body  itself  that  inter- 
fere with  its  powers  of  growth  —  yoji  poison  yourself. 
The  laws  of  food  supply  and  the  appetites  of  enemies 
are  as  inexorable  as  fate.  Life  may  be  defined  as  a 
constant  attempt  to  find  the  one  and  avoid  the  other. 
But  we  are  concerned  here  with  the  third  point,  the 
methods  that  have  been  devised  of  escape  from  the 
limitations  of  the  body  itself.  This  is  found  in  repro- 
duction. The  simplest  possible  device  is  to  divide. 
This  makes  dispersal  possible  with  an  increased  chance 
of  finding  food,  and  of  escaping  annihilation,  and  at 
the  same  time  by  reducing  the  mass  permits  of  a  more 
ready  escape  of  the  by-products  of  the  living  machine. 

Reproduction  by  simple  division  is  a  well-known  pro- 
cess in  many  of  the  lower  animals  and  plants;  it  is 
almost  universal  in  one-celled  forms,  and  not  unknown 
even  in  many-celled  organisms.  Amoeba  and  para- 
mcecium  are  the  stock  cases  for  unicellular  animals; 
many  plants  reproduce  by  buds,  tubers,  stolons,  or 
shoots ;  hydroids  and  sea-anemones  both  divide  and 
bud ;  many  planarians,  and  some  worms,  divide  trans- 
versely to  produce  two  new  individuals.  But  these 
methods  of  reproduction  are  limited  to  simple  structures 
where  concentration  and  division  of  labor  amongst  the 
organs  has  not  been  carried  to  an  extreme.  In  con- 
sequence, what  each  part  lacks  after  the  division  can  be 


4  HEREDITY  AND   SEX 

quickly  made  good,  for  delay,  if  prolonged,  would 
increase  the  chances  of  death. 

But  there  is  another  method  of  division  that  is  almost 
universal  and  is  utilized  by  high  and  by  low  forms  alike  : 
individual  cells,  as  eggs,  are  set  free  from  the  rest  of 
the  body.  Since  they  represent  so  small  a  part  of  the 
body,  an  immense  number  of  them  may  be  produced  on 
the  chance  that  a  few  will  escape  the  dangers  of  the 
long  road  leading  to  maturity.  Sometimes  the  eggs 
are  protected  by  jelly,  or  by  shells,  or  by  being  trans- 
parent, or  by  being  hidden  in  the  ground  or  under 
stones,  or  even  in  the  body  of  the  parent.  Under  these 
circumstances  the  animal  ventures  to  produce  eggs  with 
a  large  amount  of  food  stored  up  for  the  young  embryo. 

So  far  reaching  were  the  benefits  of  reproduction 
by  eggs  that  it  has  been  followed  by  almost  every 
species  in  the  animal  and  plant  kingdom.  It  is  ad- 
hered to  even  in  those  cases  where  the  animals  follow 
other  grosser  methods  of  separation  at  the  same  time. 
We  find,  however,  a  strange  limitation  has  been  put 
upon  the  process  of  reproduction  by  eggs.  Before  the 
egg  begins  its  development  it  must  be  fertilized.  Cells 
from  two  individuals  must  come  together  to  produce 
a  new  one. 

The  meaning  of  this  process  has  baffled  biologists 
ever  since  the  changes  that  take  place  during  fertili- 
zation were  first  discovered ;  in  fact,  long  before  the 
actual  processes  that  take  place  were  in  the  least  un- 
derstood. There  is  a  rather  extensive  and  antiquated 
literature  dealing  with  the  part  of  the  male  and  of 
the  female  in  the  process  of  procreation.  It  would 
take  us  too  far  to  attempt  to  deal  with  these  questions 


(TO 


, 

THE  EVOLUTION  OF  SEX  5 

in  their  historical  aspects,   but  some  of  their  most 
modern  aspects  may  well  arrest  our  attention. 

In  the  simplest  cases,  as  shown  by  some  of  the  one- 
celled  organisms,  two  individuals  fuse  into  a  single 
one  (Fig.  1) ;  in  other  related  organisms  the  two  in- 
dividuals that  fuse  may  be  unequal  in  size.  Some- 
times we  speak  of  these  as  male  and  female,  but 
it  is  questionable  whether  we  should  apply  to  these 
unicellular  types  the  same  names  that  we  use  for  the 


FIG.  1.  — •  Union  of  two  individuals  (Stephanos phcera  pluvialis)  to 

form  a  single  individual.     (After  Doflein.)  ^ 

many-celled  forms  where  the  word  sex  applies  to  the 
soma  or  body,  and  not  to  the  germ  cells. 

One  of  the  best  known  cases  of  conjugation  is  that 
of  paramcecium.  Under  certain  conditions  two  in- 
dividuals unite  and  partially  fuse  together.  An  in- 
terchange of  certain  bodies,  the  micronuclei,  then  takes 
place,  as  shown  in  Fig.  2,  and  in  diagram,  Fig.  3.  The 
two  conjugating  paramcecia  next  separate,  and  each 
begins  a  new  cycle  of  divisions.  Here  each  individual 
may  be  said  to  have  fertilized  the  other.  The  process 
recalls  what  takes  place  in  hermaphroditic  animals  of 
higher  groups  in  the  sense  that  sperm  from  one  indi- 
vidual fertilizes  eggs  of  the  other. 

We  owe  to  Maupas  the  inauguration  of  an  epoch- 
making  series  of  studies  based  on  phenomena  like  this 
in  paramcecium. 


6 


HEREDITY  AND  SEX 


FIG.  2.  —  Conjugation  in  Paramo3cium.  The  micronucleus  in  one  indi- 
vidual is  represented  in  black,  in  the  other  by  cross-lines.  The  macro- 
nucleus  in  both  is  stippled.  A-C,  division  of  micronucleus  into  2  and 
4  nuclei;  Cl-D,  elongation  of  conjugation  nuclei,  which  interchange  and 
reeombine  in  E\  F-J,  consecutive  stage  in  one  ex-conjugant  to  show  three 
divisions  of  new  micronucleus  to  produce  eight  micronuclei  (J).  In  lower 
part  of  diagram  the  first  two  divisions  of  the  ex-conjugant  (J)  with  eight 
micronuclei  are  shown,  by  means  of  which  a  redistribution  of  the  eight 
micronuclei  takes  place.  See  also  Fig.  100. 


THE  EVOLUTION  OF  SEX 


II 


ffl 


FIG.  3.  —  The  nu:-lei  of  two  individuals  of  paramcecium  in  I  (homozygous 
in  certain  factors,  and  heterozygous  in  other  factors) ,  are  represented  as  divid- 
ing twice  ( in  II  and  III ) ;  the  first  division,  II,  is  represented  as  reducing, 
i.e.  segregation  occurs  ;  the  second  division,  III,  is  represented  as  equational, 
i.e.  no  reduction  but  division  of  factors,  as  in  the  next  or  conjugation  division, 
IV,  also. 


8  HEREDITY  AND   SEX 

Maupas  found  by  following  from  generation  to 
generation  the  division  of  some  of  these  protozoa  that 
the  division  rate  slowly  declines  and  finally  comes  to 
an  end.  He  found  that  if  a  debilitated  individual 
conjugates  with  a  wild  individual,  the  death  of  the  race 
is  prevented,  but  Maupas  did  not  claim  that  through 
conjugation  the  division  rate  was  restored.  On  the 
contrary  he  found  it  is  lower  for  a  time. 

He  also  discovered  that  conjugation  between  two 
related  individuals  of  these  weakened  strains  produced 
no  beneficial  results. 

Butschlihad  earlier  (1876)  suggested  that  conjugation 
means  rejuvenation  or  renewal  of  youth,  and  Maupas' 
results  have  sometimes  been  cited  as  supporting  this 
view.  Later  work  has  thrown  many  doubts  on  this 
interpretation  and  has  raised  a  number  of  new  issues. 

In  the  first  place,  the  question  arose  whether  the 
decline  that  Maupas  observed  in  the  rate  of  division 
may  not  have  been  due  to  the  uniform  conditions  under 
which  his  cultures  were  maintained,  or  to  an  insuffi- 
ciency in  some  ingredient  of  these  cultures  rather  than 
to  lack  of  conjugation.  Probably  this  is  true,  for 
Calkins  has  shown  that  by  putting  a  declining  race 
into  a  different  medium  the  original  division  rate  may 
be  restored.  Woodruff  has  used  as  culture  media  a 
great  variety  of  food  stuffs  and  has  succeeded  in  keep- 
ing his  lines  without  loss  of  vigor  through  3000  gen- 
erations. Maupas  records  a  decline  in  other  related 
protozoa  at  the  end  of  a  few  hundred  generations. 

Butschli's  idea  that  by  the  temporary  union  (with 
interchange  of  micronuclei)  of  two  weak  individuals 
two  vigorous  individuals  could  be  produced  seems 


THE   EVOLUTION   OF   SEX  9 

mysterious ;  unless  it  can  be  made  more  explicit,  it 
does  not  seem  in  accord  with  our  physico-chemical 
conceptions.  Jennings,  who  has  more  recently  studied 
in  greater  detail  the  process  of  division  and  conjugation 
in  paramoecium,  has  found  evidence  on  which  to  base 
a  more  explicit  statement  as  to  the  meaning  of  rejuve- 
nescence through  conjugation. 

Jennings'  work  is  safeguarded  at  every  turn  by  care- 
ful controls,  and  owing  in  large  part  to  these  controls 
his  results  make  the  interpretations  more  certain.  He 
found  in  a  vigorous  race,  that  conjugated  at  rather 
definite  intervals,  that  after  conjugation  the  division 
rate  was  not  greater  than  it  had  been  before,  but  on 
the  contrary  was  slower  —  a  fact  known,  as  he  points 
out,  to  Maupas  and  to  Hertwig.  Conjugation  does 
not  rejuvenate  in  this  sense. 

Jennings  states  that,  since  his  race  was  at  the  be- 
ginning vigorous,  the  objection  might  be  raised  that 
the  conditions  were  not  entirely  fulfilled,  for  his  pred- 
ecessors had  concluded  that  it  is  a  weakened  race  that 
was  saved  from  annihilation  by  the  process.  In  order 
to  meet  this  objection  he  took  some  individuals  from 
his  stock  and  reared  them  in  a  small  amount  of  culture 
fluid  on  a  slide.  After  a  time  they  became  weakened 
and  their  rate  of  division  was  retarded.  He  then  al- 
lowed them  to  conjugate,  and  reared  the  conjugants. 
Most  of  these  were  not  benefited  in  the  leafet  by  the 
process,  and  soon  died.  A  few  improved  and  began 
to  multiply,  but  even  then  not  so  fast  as  paramcecia  hi 
the  control  cultures  that  had  been  prevented  from  con- 
jugating. Still  others  gave  intermediate  rates  of 
division. 


10 


HEREDITY  AND  SEX 


He  concludes  that  conjugation  is  not  in  itself  bene- 
ficial to  all  conjugants,  but  that  the  essence  of  the  pro- 
cess is  that  a  recombination  of  the  hereditary  traits 
occurs  as  shown  in  the  diagram,  Fig.  3  and  4.  Some 


FIG.  4.  —  Illustrating  conjugation  between  two  stocks,  with  pairs  of 
factors  A,  B,  C,  D,  and  a,  b,  c,  d  ;  and  union  of  pairs  into  Aa,  Bb,  Cc,  Dd. 
After  these  separate,  their  possible  recombinations  are  shown  in  the  16 
smaller  circles.  (After  Wilson.). 

of  these  new  combinations  are  beneficial  for  special 
conditions  —  others  not.  The  offspring  of  those  con- 
jugants that  have  made  favorable  combinations  will 
soon  crowd  out  the  descendants  of  other  conjugants 
that  have  made  mediocre  or  injurious  combinations. 
Hence,  in  a  mass  culture  containing  at  all  times  large 


THE   EVOLUTION  OF  SEX 


11 


numbers  of  individuals,  the  maximum  division  rate  is 
kept  up,  because,  at  any  one  time,  the  majority  of  the 
individuals  come  from  the  combinations  favorable  to 
that  special  environment. 

There  are  certain  points  in  this  argument  that  call 
for  further  consideration.  In  a  mass  culture  the  fa- 
vorable combinations  for  that  culture  will  soon  be  made, 
if  conjugation  is  taking  place.  At  least  this  is  true  if 
such  combinations  are  homogeneous  (homozygous,  in 
technical  language).  Under  such  circumstances  the 
race  will  become  a  pure  strain,  and  further  conjugation 
could  do  nothing  for  it  even  if  it  were  transferred  to  a 
medium  unsuited  to  it. 

In  the  ordinary  division  of  a  cell  every  single  de- 
terminer divides  and  each  of  the  new  cells  receives 
half  of  each  determiner.  Hence  in  the  case  of  para- 
mcecium  all  the  descendants  of  a  given  paramoecium 
that  are  produced  by  division  must  be  exactly  alike. 
But  in  preparation  for  conjugation  a  different  pro- 
cess may  be  supposed  to  take  place,  as  in  higher 
animals,  among  the  determiners.  The  determiners 
unite  in  pairs  and  then,  by  division,  separate  from 
each  other,  Fig.  4.  In  consequence  the  number  of 
determiners  is  reduced  to  half.  Each  group  of  deter- 
miners will  be  different  from  the  parent  group,  pro- 
vided the  two  determiners  that  united  were  not 
identical.  If  after  this  has  occurred  conjugation 
takes  place,  the  process  not  only  restores  the  total 
number  of  determiners  in  each  conjugant,  but  gives 
new  groups  that  differ  from  both  of  the  original 
groups. 

The   maintenance   of   the   equilibrium   between   an 


12  HEREDITY  AND   SEX 

organism  and  its  environment  must  be  a  very  delicate 
matter.  One  combination  may  be  best  suited  to  one 
environment,  and  another  combination  to  another. 
Conjugation  brings  about  in  a  population  a  vast  num- 
ber of  combinations,  some  of  which  may  be  suited  to 
the  time  and  place  where  they  occur.  These  survive 
and  produce  the  next  generation. 

Jennings'  experiments  show,  if  I  understand  him 
correctly,  that  the  race  he  used  was  not  homogeneous 
in  its  hereditary  elements ;  for  when  two  individuals 
conjugated,  new  combinations  of  the  elements  were 
form  3d.  It  seems  probable,  therefore,  that  the  chemi- 
cal equilibrium  of  paramcecium  is  maintained  by  the 
presence  of  not  too  much  of  some,  or  too  little  of  other, 
hereditary  materials.  In  a  word,  its  favorable  com- 
binations are  mixed  or  heterozygous. 

The  meaning  of  conjugation,  and  by  implication, 
the  meaning  of  fertilization  in  higher  forms  is  from  this 
point  of  view  as  follows  :  —  In  many  forms  the  race,  as  a 
whole,  is  best  maintained  by  adapting  itself  to  a  widely 
varied  environment.  A  heterozygous  or  hybrid  con- 
stitution makes  this  possible,  and  is  more  likely  to 
perpetuate  itself  in  the  long  run  than  a  homozygous 
race  that  is  from  the  nature  of  the  case  suited  to  a  more 
limited  range  of  external  conditions. 

What  bearing  has  this  conclusion  on  the  problem  of 
the  evolution  of  sex  and  of  sexual  reproduction  ? 

This  is  a  question  that  is  certain  to  be  asked.  I  am 
not  sure  that  it  is  wise  to  try  to  answer  it  at  present, 
in  the  first  place  because  of  the  uncertainty  about  the 
conclusions  themselves,  and  in  the  next  place,  because, 
personally,  I  think  it  very  unfair  and  often  very  unfor- 


THE  EVOLUTION  OF  SEX  13 

tunate  to  measure  the  importance  of  every  result  by 
its  relation  to  the  theory  of  evolution.  But  with  this 
understanding  I  may  venture  upon  a  few  suggestions. 

If  a  variation  should  arise  in  a  hermaphroditic 
species  (already  reproducing  sexually)  that  made  cross- 
fertilization  more  likely  than  self-fertilization,  and  if, 
as  a  rule,  the  hybrid  condition  (however  this  may  be 
explained)  is  more  vigorous  in  the  sense  that  it  leaves 
more  offspring,  such  a  variation  would  survive,  other 
things  being  equal. 

But  the  establishment  of  the  contrivance  in  the 
species  by  means  of  which  it  is  more  likely  to  cross- 
fertilize,  might  in  another  sense  act  as  a  drawtack. 
Should  weak  individuals  appear,  they,  too,  may  be 
perpetuated,  for  on  crossing,  their  weakness  is  concealed 
and  their  offspring  are  vigorous  owing  to  then-  hybrid 
condition.  The  race  will  be  the  loser  in  so  far  as  re- 
cessive or  weak  combinations  will  continue  to  appear, 
as  they  do  in  many  small  communities  that  have  some 
deficiency  hi  their  race ;  but  it  is  a  question  whether  the 
vigor  that  comes  from  mixing  may  not  more  than  com- 
pensate for  the  loss  due  to  the  continual  appearance  of 
weakened  individuals. 

This  argument  applies  to  a  supposed  advantage 
within  the  species.  But  recombination  of  what  already 
exists  will  not  lead  to  the  development  of  anything 
that  is  essentially  new.  Evolution,  however,  is  con- 
cerned with  the  appearance  and  maintenance  of  new 
characters.  Admitting  that  sexual  reproduction  proved 
an  advantage  to  species,  and  especially  so  when  com- 
bined with  a  better  chance  of  cross-fertilization,  the 
machinery  would  be  at  hand  by  means  of  which  any 


14  HEREDITY   AND   SEX 

new  character  that  appeared  would  be  grafted,  so  to 
speak,  on  to  the  body  of  the  species  in  which  it  appeared. 
Once  introduced  it  would  be  brought  into  combination 
with  all  the  possible  combinations,  or  races,  already 
existing  within  the  species.  Some  of  the  hybrid  com- 
binations thus  formed  might  be  very  vigorous  and  would 
survive.  This  reasoning,  while  hypothetical,  and,  per- 
haps not  convincing,  points  at  least  to  a  way  in 
which  new  varieties  may  become  incorporated  into 
the  body  of  a  species  and  assist  in  the  process  of 
evolution. 

It  might  be  argued  against  this  view  that  the  same 
end  would  be  gained,  if  a  new  advantageous  variation 
arose  in  a  species  that  propagated  by  non-sexual 
methods  or  in  a  species  that  propagated  by  self-fertili- 
zation. The  offspring  of  such  individuals  would  trans- 
mit their  new  character  more  directly  to  the  offspring. 
Evolution  may,  of  course,  at  times  have  come  about 
in  this  way,  and  it  is  known  that  in  many  plants  self- 
fertilization  is  largely  or  exclusively  followed.  But  in 
a  species  in  which  cross-fertilization  was  the  estab- 
lished means  of  propagation,  the  new  character  would 
be  brought  into  relation  with  all  the  other  variations 
that  are  found  in  the  component  races 'and  increase 
thereby  its  chances  of  favorable  combinations.  We 
have  in  recent  years  come  to  see  that  a  new  heritable 
character  is  not  lost  by  crossing,  or  even  weakened  by 
" blending,"  as  was  formerly  supposed  to  be  the  case; 
hence  no  loss  to  the  character  itself  will  result  in  the 
union  with  other  strains,  or  races,  within  the  species. 

If  then  we  cannot  explain  the  origin  of  sexual  re- 
production by  means  of  the  theory  of  evolution,  we 


THE   EVOLUTION   OF  SEX  15 

can  at  least  see  how  the  process  once  begun  might  be 
utilized  in  the  building  up  of  new  combinations ;  and 
to-day  evolution  has  come  to  mean  not  so  much  a 
study  of  the  origination  of  new  characters  as  the  method 
by  which  new  characters  become  established  after  they 
have  appeared. 

THE    BODY   AND    THE    GERM-PLASM 

As  I  have  said,  it  is  not  unusual  to  speak  of  the  uni- 
cellular animals  and  plants  as  sexual  individuals,  and 
where  one  of  them  is  larger  than  the  other  it  is  some- 
times called  the  female  and  the  smaller  the  male.  But 
in  many-celled  animals  we  mean  by  sex  something 
different,  for  the  term  applies  to  the  body  or  soma,  and 
not  to  the  reproductive  cells  at  all.  The  reproductive 
cells  are  eggs  and  sperm.  It  leads  to  a  good  deal  of 
confusion  to  speak  of  the  reproductive  cells  as  male 
and  female.  In  the  next  chapter  it  will  be  pointed  out 
that  the  eggs  and  sperm  carry  certain  materials ;  and 
that  certain  combinations  of  these  materials,  after  fer- 
tilization has  occurred,  produce  females ;  other  combi- 
nations produce  males ;  but  males  and  females,  as  such, 
do  not  exist  until  after  fertilization  has  taken  place. 

The  first  step,  then,  in  the  evolution  of  sex  was  taken 
when  colonies  of  many  cells  appeared.  We  find  a 
division  of  labor  in  these  many-celled  organisms ;  the 
germ-cells  are  hidden  away  inside  and  are  kept  apart 
from  the  wear  and  tear  of  life.  Their  maintenance 
and  protection  are  taken  over  by  the  other  cells  of  the 
colony.  Even  among  the  simplest  colonial  forms  we 
find  that  some  colonies  become  specialized  for  the  pro- 
duction of  small,  active  germ-cells.  These  colonies 


16  HEREDITY  AND   SEX 

are  called  the  males,  or  sperm-producing  colonies.     The 
other  colonies  specialize  to  produce  larger  germ-cells  — 
the  eggs.     These  colonies  are  called  females  or  egg-pro- 
ducing colonies.     Sex  has  appeared  in  the  living  world. 

To-day  we  are  only  beginning  to  appreciate  the  far- 
reaching  significance  of  this  separation  into  the  immor- 
tal germ-cells  and  the  mortal  body,  for  there  emerges 
the  possibility  of  endless  relations  between  the  body  on 
the  one  hand  and  the  germ-cells  on  the  other.  What- 
ever the  body  shows  in  the  way  of  new  characters 
or  new  ways  of  reacting  must  somehow  be  represented 
in  the  germ-cells  if  such  characters  are  to  be  perpetu- 
ated. The  germ-cells  show  no  visible  modification  to 
represent  their  potential  characters.  Hence  the  classi- 
cal conundrum  —  whether  the  hen  appeared  before  the 
egg,  or  the  egg  before  the  hen  ?  Modern  biology  has 
answered  the  question  with  some  assurance.  The  egg 
came  first,  the  hen  afterwards,  we  answer  dogmati- 
cally, because  we  can  understand  how  any  change  in 
the  egg  will  show  itself  in  the  next  generation  —  in 
the  new  hen,  for  instance ;  but  despite  a  vast  amount 
of  arguing  no  one  has  shown  how  a  new  hen  could  get 
her  newness  into  the  old-fashioned  eggs. 

Few  biological  questions  have  been  more  combated 
than  this  attempt  to  isolate  the  germ-tract  from  the 
influence  of  the  body.  Nussbaum  was  amongst  the 
first,  if  not  the  first,  to  draw  attention  to  this  distinc- 
tion, but  the  credit  of  pointing  out  its  importance  is 
generally  given  to  Weismann,  whose  fascinating  specu- 
lations start  from  this  idea.  For  Weismann,  the  germ- 
cells  are  immortal  —  the  soma  alone  has  the  stigma  of 
death  upon  it.  Each  generation  hands  to  the  next 


THE   EVOLUTION  OF  SEX  17 

one  the  immortal  stream  unmodified  by  the  experience 
of  the  body.  What  we  call  the  individual,  male  or 
female,  is  the  protecting  husk.  In  a  sense  the  body  is 
transient  —  temporary.  Its  chief  " purpose"  is  not 
its  individual  life,  so  much  as  its  power  to  support  and 
carry  to  the  next  point  the  all  important  reproductive 
material. 

Modern  research  has  gone  far  towards  establishing 
Weismann's  claims  in  this  regard.  It  is  true  that  the 
germ-plasm  must  sometimes  change  —  otherwise  there 
could  be  no  evolution.  But  the  evidence  that  the  germ- 
plasm  responds  directly  to  the  experiences  of  the  body 
has  no  substantial  evidence  in  its  support.  I  know,  of 
course,  that  the  whole  Lamarckian  school  rests  its 
argument  on  the  assumption  that  the  germ-plasm  re- 
sponds to  all  profound  changes  in  the  soma ;  but  despite 
the  very  large  literature  that  has  grown  up  dealing  with 
this  matter,  proof  is  still  lacking.  And  there  is  abun- 
dant evidence  to  the  contrary. 

On  the  other  hand,  there  is  evidence  to  show  that 
the  germ-plasm  does  sometimes  change  or  is  changed. 
Weismann's  attempt  to  refer  all  such  changes  to  recom- 
binations of  internal  factors  in  the  germ-plasm  it- 
self has  not  met  with  much  success.  Admitting  that 
new  combinations  may  be  brought  about  in  this 
way,  as  explained  for  paramcecium,  yet  it  seems  un- 
likely that  the  entire  process  of  evolution  could  have 
resulted  by  recombining  what  already  existed;  for 
it  would  mean,  if  taken  at  its  face  value,  that  by  re- 
combination of  the  differences  already  present  in  the 
first  living  material,  all  of  the  higher  animals  and  plants 
were  foreordained.  In  some  way,  therefore,  the  germ- 


18  HEREDITY  AND   SEX 

plasm  must  have  changed.  We  have  then  the  alter- 
natives. Is  there  some  internal,  initial  or  driving  im- 
pulse that  has  led  to  the  process  of  evolution  ?  Or  has 
the  environment  brought  about  changes  in  the  germ- 
plasm  ?  We  can  only  reply  that  the  assumption  of  an 


•§• 


FIG.  5.  —  Schematic  representation  of  the  processes  occurring  during 
the  fertilization  and  subsequent  segmentation  of  the  ovum.  (Boveri,  from 
Howell.) 

internal  force  puts  the  problem  beyond  the  field  of 
scientific  explanation.  On  the  other  hand,  there  is  a 
small  amount  of  evidence,  very  incomplete  and  in- 
sufficient at  present,  to  show  that  changes  in  the  en- 
vironment reach  through  the  soma  and  modify  the 
germinal  material. 


THE  EVOLUTION  OF  SEX  19 

It  would  take  us  too  far  from  our  immediate  subject 
to  attempt  to  discuss  this  matter,  but  it  has  been  nec- 
essary to  refer  to  it  in  passing,  for  it  lies  at  the  founda- 
tion of  all  questions  of  heredity  and  even  involves,  as 
we  shall  see  later,  the  question  of  heredity  of  sex. 

This  brings  us  back  once  more  to  the  provisional 
conclusion  we  reached  in  connection  with  the  experi- 
ments on  paramcecium.  When  the  egg  is  fertilized 
by  the  sperm,  Fig.  5,  the  result  is  essentially  the  same 
as  that  which  takes  place  when  two  paramoecia  fer- 
tilize each  other.  The  sperm  brings  into  the  egg  a 
nucleus  that  combines  with  the  egg-nucleus.  The  new 
individual  is  formed  by  recombining  the  hereditary 
traits  of  its  two  parents. 

It  is  evident  that  fertilization  accomplishes  the  same 
result  as  conjugation.  If  our  conclusion  for  paramce- 
cium holds  we  can  understand  how  animals  and  plants 
with  eggs  and  sperm  may  better  readjust  themselves 
now  to  this,  now  to  that  environment,  within  certain 
limits.  But  we  cannot  conclude,  as  I  have  said,  that  this 
process  can  make  any  permanent  contribution  to  evolu- 
tion. It  is  true  that  Weismann  has  advanced  the  hy- 
pothesis that  such  recombinations  furnish  the  materials 
for  evolution,  but  as  I  have  said  there  is  no  evidence 
that  supports  or  even  makes  plausible  his  contention. 

I  bring  up  again  this  point  to  emphasize  that  while  the 
conclusion  we  arrived  at  —  a  provisional  conclusion  at 
best  —  may  help  us  to  understand  how  sexual  repro- 
duction might  be  beneficial  to  a  species  in  maintaining 
itself,  it  cannot  be  utilized  to  explain  the  progressive 
advances  that  we  must  believe  to  have  taken  place 
during  evolution. 


20  HEREDITY  AND   SEX 

THE    EARLY   ISOLATION    OF   THE    GERM-CELLS 

There  is  much  evidence  to  show  that  the  germ-cells 
appear  very  early  in  the  development  of  the  individual 
when  they  are  set  aside  from  the  cells  that  differentiate 
into  the  body  cells.  This  need  not  mean  that  the  germ- 
cells  have  remained  unmodified,  although  this  is  at 


FIG.  6.  —  Chromatin  diminution  and  origin  of  the  germ-cells  in  Ascaris. 
(After  Boveri.) 


THE   EVOLUTION   OF  SEX 


21 


first  sight  the  most  natural  interpretation.  It  might  be 
said,  indeed,  that  they  are  among  the  first  cells  to 
differentiate,  but  only  in  the  sense  that  they  specialize, 
as  germ-cells. 


FIG.  7.  —  Origin  of  germ-cells  in  Sagitta. 
Korschelt  and  Heider.) 


(From 


In  a  parasitic  worm,  ascaris,  one  of  the  first  four 
cells  divides  differently  from  the  other  three  cells.  As 
seen  in  Fig.  6,  this  cell  retains  at  its  division  all  of  its 
chromatin  material,  while  in  the  other  three  cells  some 
of  the  chromatin  is  thrown  out  into  the  cell-plasm.  The 


''-*'•'*£•••& 


i 


:'M< 


o 


FIG.  8.  —  Origin  of  germ-cells  in  Miastor.  Note  small  black  proto- 
plasmic area  at  bottom  of  egg  into  which  one  of  the  migrating  segmentation 
nuclei  moves  to  produce  the  germ-cells.  (After  Kahle.) 


22 


HEREDITY  AND   SEX 


single  cell  that  retains  all  of  the  chromatin  in  its  nucleus 
gives  rise  to  the  germ-cells. 

In  a  marine  worm-like  form,  sagitta,  two  cells  can 
easily  be  distinguished  from  the  other  cells  in  the  wall  of 
the  digestive  tract  (Fig.  7).  They  leave  their  first  posi- 
tion and  move  into  the  interior  of  the  body,  where  they 
produce  the  ovary  and  testes. 


FIG.  9.  —  Origin  of  germ-cells  in  certain  vertebrates,  viz.  turtle,  frog, 
gar-pike  and  bow-fin.  The  germ-cells  as  darker  cells  are  seen  migrating  from 
the  digestive  tract  (endoderm).  (After  Allen.) 

In  several  of  the  insects  it  has  been  shown  that  at  a 
very  early  stage  in  the  segmentation,  one,  or  a  few  cells 
at  most,  lying  at  one  end  of  the  egg  develop  almost  in- 
dependently of  the  rest  of  the  embryo  (Fig.  8).  Later 
they  are  drawn  into  the  interior,  and  take  up  their 
final  location,  where  they  give  rise  to  the  germ-cells. 

Even  in  the  vertebrates,   where,   according  to  the 


THE  EVOLUTION  OF  SEX  23 

earlier  accounts,  the  germ-cells  were  described  as  appear- 
ing late  in  embryonic  development,  it  has  been  shown 
that  the  germ-cells  can  be  detected  at  a  very  early  stage 
in  the  walls  of  the  digestive  tract  (Fig.  9).  Thence  they 
migrate  to  their  definitive  position,  and  give  rise  to 
the  cells  from  which  the  eggs  or  the  sperm  arise. 

The  germ-cells  are  in  fact  often  the  earliest  cells  to 
specialize  in  the  sense  that  they  are  set  aside  from  the 
other  cells  that  produce  the  soma  or  body  of  the  in- 
dividual. 

THE  APPEARANCE  OF  THE  ACCESSORY  ORGANS  OF 
REPRODUCTION 

As  animals  became  larger  the  problem  of  setting  free 
the  germ-cells  was  a  matter  of  great  importance.  Sys- 
tems of  outlets  arose  —  the  organism  became  piped,  as  it 
were.  In  the  lower  animals  the  germ-cells  are  brought 
to  the  surface  and  set  free  directly,  and  fertilization  is  a 
question  of  the  chance  meeting  of  sperm  and  egg ;  for 
there  is  practically  no  evidence  to  show  that  the  sperm 
is  attracted  to  the  egg  and  much  evidence  that  it  is 
not.  Later,  the  copulatory  organs  were  evolved  in  all 
the  higher  groups  of  animals  by  means  of  which  the 
sperm  of  the  male  is  transferred  directly  to  the  female. 
This  makes  more  certain  the  fertilization  of  the  egg. 

In  the  mollusks,  in  the  insects  and  crustaceans,  and 
in  the  vertebrates  the  organs  of  copulation  serve  to 
hold  the  individuals  together  during  the  act  of  mating, 
and  at  the  same  time  serve  to  transfer  the  semen  of  the 
male  to  the  oviduct,  or  to  special  receptacles  of  the 
female.  Highly  elaborated  systems  of  organs  and 
special  instincts,  no  less  elaborate,  serve  to  make  the 


24 


HEREDITY  AND   SEX 


union  possible.  In  some  types  mating  must  occur  for 
each  output  of  eggs,  but  in  other  cases  the  sperm  is 
stored  up  in  special  receptacles  connected  with  the  ducts 
of  the  female.  From  these  receptacles  a  few  sperm  at 
a  time  may  be  set  free  to  fertilize  each  egg  as  it  passes 
the  opening  of  the  receptaculum.  In  the  queen  bee 
enough  sperm  is  stored  up  to  last  the  queen  for  five  or 
six  years  and  enough  to  fertilize  a  million  eggs. 


FIG.  10.  —  Squid  :  Two  upper  right-hand  figures  illustrate  two  methods 
of  copulation.  Lower  right-hand  figure  dissected  to  show  spermatophore 
placed  in  mantle  cavity  of  female.  Left-hand  figure  (below),  spermatophore 
pocket  behind  mouth  of  male;  upper  figure,  section  of  same.  (After  Drew.) 

There  are  a  few  cases  where  the  transfer  from  the 
male  to  the  female  is  brought  about  in  a  different  way. 
The  most  striking  cases  are  those  of  the  squids  and 
octopi,  and  of  the  spiders. 

In  the  squid,  the  male  and  female  interlock  arms 
(Fig.  10).  The  male  takes  the  packets  of  sperm  (that 
are  emitted  at  this  time  from  the  sperm-duct)  by  means 
of  a  special  arm,  and  transfers  the  packets  either  to  a 


THE   EVOLUTION   OF  SEX 


25 


special  receptacle  within  the  circle  of  arms  of  the  female, 
or  plants  them  within  the  mantle  chamber  itself  of  the 
female.  Each  packet  of  spermatozoa  is  contained  in  a 
long  tube.  On  coming  in  contact  with  sea  water  the 
tube  everts  at  one  end,  and  allows  the  sperm  to  escape. 


FIG.   11.  —  Octopus,  male  showing  hectocotyl  arm  (ha).     Cop- 
ulation (below),  small  male,  A;  large  female,  B. 

After  separation  the  female  deposits  her  strings  of 
eggs,  which  are  fertilized  by  the  sperm  escaping  from 
the  spermatophores.  In  octopus  and  its  allies,  one 
arm,  that  is  used  to  transfer  the  spermatophores,  is 
specially  modified  at  the  breeding  season  (Fig.  11). 


26  HEREDITY  AND  SEX 

This  arm  is  inserted  by  the  male,  as  shown  in  the  figure, 
within  the  mantle  chamber  of  the  female.  In  some 
species,  Argonauta  argo  for  instance  (Fig.  12),  the  arm 


FIG.  12.  —  Argonauta  showing  developing  (A)  and  developed  (B) 
hectocotyl  arm,  which,  after  being  charged  with  spermatophores,  is  left  in 
mantle  of  female. 

is  broken  off,  and  remains  attached  by  its  suckers  in- 
side the  mantle  of  the  female.  The  eggs  are  later  fer- 
tilized by  sperm  set  free  from  this  "  hectocotylized  "  arm. 

THE    SECONDARY    SEXUAL    CHARACTERS 

In  the  most  highly  evolved  stages  in  the  evolution 
of  sex  a  new  kind  of  character  makes  its  appearance. 
This  is  the  secondary  sexual  character.  In  most  cases 
such  characters  are  more  elaborate  in  the  male,  but 
occasionally  in  the  female.  They  are  the  most  aston- 
ishing thing  that  nature  has  done :  brilliant  colors, 
plumes,  combs,  wattles,  and  spurs,  scent  glands  (pleas- 
ant and  unpleasant) ;  red  spots,  yellow  spots,  green 
spots,  topknots  and  tails,  horns,  lanterns  for  the  dark, 
songs,  howlings,  dances  and  tourneys  —  a  medley  of 
odds  and  ends. 

The  most  familiar  examples  of  these  characters  are 
found  in  vertebrates  and  insects,  while  in  lower  forms 


THE  EVOLUTION   OF  SEX  27 

they  are  rare  or  absent  altogether.  In  mammals  the 
horns  of  the  male  stag  are  excellent  examples  of  second- 
ary sexual  characters.  The  male  sea  cow  is  much 
greater  in  size  than  the  female,  and  possesses  long  tusks. 
The  mane  of  the  lion  is  absent  in  the  lioness. 


FIG.  13.  —  Great  bird  of  Paradise,  male  and  female. 
(After  Elliot.) 

In  birds  there  are  many  cases  in  which  the  sexes  differ 
in  color  (Figs.  13  and  14).  The  male  is  often  more 
brilliantly  colored  than  the  female  and  in  other  cases 
the  nuptial  plumage  of  the  male  is  quite  different  from 
the  plumage  of  the  female.  For  example,  the  black 
and  yellow  colors  of  the  male  bobolink  are  in  striking 
contrast  with  the  brown-streaked  female  (Fig.  15). 
The  male  scarlet  tanager  has  a  fiery  red  plumage  with 
black  wings,  while  the  female  is  olive  green.  The  male 


28 


HEREDITY  AND   SEX 


of  the  mallard  duck  has  a  green  head  and  a  reddish 
breast  (Fig.  16),  while  the  female  is  streaked  with  brown. 
In  insects  the  males  of  some  species  of  beetles  have 
horns  on  the  head  that  are  lacking  in  the  female  (Fig. 
17).  The  males  of  many  species  of  butterflies  are  col- 
ored differently  from  the  females. 


FIG.   14.  —  White-booted  humming  bird,  two  males 
and  one  female.     (After  Gould.) 

The  phosphorescent  organ  of  our  common  firefly, 
Photinus  pyralis,  is  a  beautiful  illustration  of  a  second- 
ary sexual  character.  On  the  under  surface  of  the  male 
there  are  two  bands  and  of  the  female  there  is  a  single 
band  that  can  be  illuminated  (Fig.  18).  At  night  the 
males  leave  their  concealment  and  fly  about.  A  little 
later  the  females  ascend  to  the  tops  of  blades  of  grass 


THE  EVOLUTION  OF  SEX 


29 


s 


FIG.   15.  —  Male  and  female  bobolink.     (From 
"Bird  Lore.") 


FIG.   16.  —  Male  and  female  mallard  duck.     (From 
"  Bird  Lore.") 


30  HEREDITY  AND   SEX 

and  remain  there  without  glowing.  A  male  passes  by 
and  flashes  his  light ;  the  female  flashes  back.  In- 
stantly he  turns  in  his  course  to  the  spot  whence  the 
signal  came  and  alights.  He  signals  again.  She  re- 
plies. He  ascends  the  blade,  and  if  he  cannot  find  her, 
he  signals  again  and  she  responds.  The  signals  con- 


FIG.  17.  —  Male  and  female  Hercules 
beetle.     (After  Kingsley.) 

tinue  until  the  female  is  found,  and  the  drama  of  sex 
is  finished. 

Mast  has  recently  shown  that  the  female  firefly  does 
more  than  simply  respond  to  the  signal  of  the  male. 
If  a  male  flies  above  and  to  the  right  of  the  female,  she 
bends  her  abdomen  so  that  its  ventral  surface  is  turned 
upward  and  to  the  right.  If  the  male  is  above  and  to 
the  left,  the  light  is  turned  in  this  direction.  If  the  male 


THE  EVOLUTION   OF  SEX  31 

is  directly  above,  the  abdomen  of  the  female  is  twisted 
almost  upward.  But  if  the  male  is  below  her,  she  emits 
her  light  without  turning  the  body.  In  the  firefly  the 
evidence  that  the  phosphorescent  organ  is  of  use  in 
bringing  the  sexes  together  seems  well  established. 


FIG.   18.  —  Male  and  female  firefly. 

Whether  all  secondary  sexual  organs  are  useful  in 
mating  is  a  question  that  must  be  referred  to  a  later 
chapter. 

THE    SEXUAL   INSTINCTS 

Side  by  side  with  the  evolution  of  these  many  kinds 
of  structural  difference  the  sexual  instincts  have  evolved. 
It  is  only  in  the  lowest  forms  that  the  meeting  of  the 
egg  and  sperm  is  left  to  chance.  The  instincts  that 
bring  the  males  and  females  together  at  the  mating 
season,  the  behavior  of  the  individuals  at  this  time  in 


32  HEREDITY  AND   SEX 

relation  to  each  other,  forms  one  of  the  most  curious 
chapters  in  the  evolution  of  sex,  for  it  involves  court- 
ship between  the  males  and  females;  the  pairing  or 
union  of  the  sexes  and  subsequently  the  building  of 
the  nest,  the  care,  the  protection  and  feeding  of  the 
young,  by  one  or  both  parents.  The  origin  of  these 
types  of  behavior  is  part  of  the  process  of  evolution  of 
sex ;  the  manner  of  their  transmission  in  heredity  and 
their  segregation  according  to  sex  is  one  of  the  most 
difficult  questions  in  heredity  —  one  about  which  noth- 
ing was  known  until  within  recent  years,  when  a 
beginning  at  least  has  been  made. 

A  few  samples  taken  almost  at  random  will  illustrate 
some  of  the  familiar  features  in  the  psychology  of  sex. 
Birds  have  evolved  some  of  the  most  complicated  types 
of  courtship  that  are  known.  It  is  in  this  group,  too, 
as  we  have  seen,  that  the  development  of  secondary 
sexual  characters  has  reached  perhaps  its  highest  types. 
But  it  is  not  necessarily  in  the  species  that  have  the 
most  striking  differences  between  the  sexes  that  the 
courtship  is  most  elaborate.  In  pigeons  and  their 
allies,  for  example,  the  courtship  is  prolonged  and  elab- 
orate, yet  the  males  and  females  are  externally  al- 
most indistinguishable  ;  while  in  the  barnyard  fowl  and 
in  ducks  the  process  is  relatively  simple,  yet  chanti- 
cleer is  notoriously  overdressed. 

Even  in  forms  so  simply  organized  as  the  fishes  it  is 
known  that  the  sexual  instincts  are  well  developed. 
In  the  common  minnow,  fundulus,  the  males  develop  in 
the  breeding  season  elaborate  systems  of  tactile  organs. 
The  male  swims  by  the  side  of  the  female,  pressing 
his  body  against  her  side,  which  causes  her  to  set  free 


THE   EVOLUTION   OF  SEX  33 

a  few  eggs.  At  the  same  time  the  male  sets  free  the 
sperm,  thereby  increasing  the  chance  that  some  of  the 
spermatozoa  will  reach  the  egg. 

In  bees,  the  sexual  life  of  the  hive  is  highly  special- 
ized. Mating  never  occurs  in  the  hive,  but  when  the 
young  queen  takes  her  nuptial  flight  she  is  followed  by 
the  drones  that  up  to  this  have  led  an  indolent  and  use- 
less life  in  the  colony.  Mating  occurs  high  in  the  air. 
The  queen  goes  to  the  new  nest  and  is  followed  by  a 
swarm  of  workers  who  construct  for  her  a  new  home. 
Here  she  remains  for  the  rest  of  her  life,  fed  and  cared 
for  by  the  workers,  who  give  her  the  most  assiduous 
attention  —  an  attention  that  might  be  compared  to 
courting  were  it  not  that  the  workers  are  not  males 
but  only  immature  females.  The  occurrence  of  these 
instincts  in  the  workers  that  never  leave  or  rarely  at 
least  leave  offspring  of  their  own  is  a  special  field  of 
heredity  about  which  we  can  do  little  more  than  specu- 
late. This  much,  however,  may  be  hazarded.  The 
inheritance  of  the  queen  and  of  the  worker  is  the  same. 
We  know  from  experimental  evidence  that  the  amount 
of  food  given  to  the  young  grub,  when  it  hatches  from 
the  egg,  is  the  external  agent  that  makes  the  grub  a 
queen  or  a  worker.  In  the  worker  the  sex  glands  are 
little  developed.  Possibly  their  failure  to  develop  may 
in  part  account  for  the  different  behavior  of  the  workers 
and  of  the  queen.  I  shall  devote  a  special  chapter  to 
this  question  of  the  influence  of  the  secretions  of  the 
sex  glands  or  reproductive  organs  on  the  character  of 
the  body.  We  shall  see  that  in  some  animals  at  least 
an  important  relation  exists  between  them. 

In  the  spiders  the  mating  presents  a  strange  spectacle. 


34  HEREDITY  AND   SEX 

Let  us  follow  Montgomery's  careful  observations  on 
Phidippus  purpuratus.  The  male  spun  a  small  web 
of  threads  from  the  floor  to  one  side  of  his  cage  at  an 
angle  of  45°.  "Four  minutes  later  he  deposited  a 
minute  drop  of  sperm  on  it,  barely  visible  to  the  naked 
eye ;  then  extending  his  body  over  the  web  reached  his 
palpi  downwards  and  backwards,  applying  them  al- 
ternately against  the  drop ;  the  palpal  organs  were 
pressed,  not  against  the  free  surface  of  the  drop,  but 
against  the  other  side  of  the  web."  Later,  a  minute 
drop  of  sperm  is  found  sticking  to  the  apex  of  one  of  the 
palpi.  In  1678  Lister  had  shown  that  the  male  applies 
his  palpi  to  the  genital  aperture  of  the  female  ;  but  not 
until  1843  was  it  found  by  Menge  that  the  palpi  carry 
the  sperm  drop. 

In  man,  courtship  may  be  an  involved  affair.  Much 
of  our  literature  revolves  about  this  period,  while  paint- 
ing and  sculpture  take  physical  beauty  as  their  theme. 
Unsatiated  with  the  natural  differences  that  distinguish 
the  sexes,  man  adds  personal  adornment  which  reaches 
its  climax  in  the  period  of  courtship,  and  leaves  a 
lasting  impression  on  the  costuming  of  the  sexes. 
Nowhere  in  the  animal  kingdom  do  we  find  such  a 
mighty  display;  and  clothes  as  ornaments  excel  the 
most  elaborate  developments  of  secondary  sexual  char- 
acters of  creatures  lower  in  the  scale. 

I  have  sketched  in  briefest  outline  some  of  the  gen- 
eral and  more  familiar  aspects  of  sex  and  the  evolution 
of  the  sexes.  In  the  chapters  that  follow  we  shall  take 
up  in  greater  detail  many  of  the  problems  that  have 
been  only  touched  upon  here. 


CHAPTER  II 
THE  MECHANISM  OF  SEX-DETERMINATION 

IN  many  species  of  animals  and  plants  two  kinds  of 
individuals  are  produced  in  every  generation.  This 
process  occurs  with  such  regularity  and  persistence  that 
our  minds  naturally  seek  some  mechanism,  some  sort 
of  orderly  machinery,  by  which  this  condition  is  brought 
about.  Yet  from  the  time  of  Aristetle  alniQst  to  the 
present  day  the  problem  has  baffled  completely  all 
attempts  at  its  solution.  However,  the  solution  is  very 
simple.  Now  that  we  hold  the  situation  in  our  grasp, 
it  seems  surprising  that  no  one  was  keen  enough  to 
deduce  it  by  purely  theoretical  reasoning.  At  least 
the  general  principles  involved  might  have  been  de- 
duced, although  we  can  see  that  without  an  intimate 
knowledge  of  the  changes  that  take  place  in  the  germ- 
cells  the  actual  mechanism  could  never  have  been 
foretold. 

The  bodies  of  animals  and  plants  are  composed  of 
millions  of  protoplasm-filled  compartments  that  are 
called  cells.  In  the  middle  of  each  cell  there  is  a  sphere, 
or  nucleus,  containing  filaments  called  chromosomes 
(Fig.  5). 

At  each  division  of  a  cell  the  wall  of  the  nucleus  is 
absorbed,  and  the  thread-like  chromosomes  contract 
into  rod-shaped,  or  rounded  bodies  (Fig.  6).  Each 
chromosome  splits  lengthwise  into  halves ;  the  halves 

35 


36  HEREDITY  AND  SEX 

are  brought  into  relation  with  a  spindle-shaped  system 
of  lines,  and  move  apart  along  these  lines  to  opposite 
sides  of  the  cell.  The  protoplasm  of  the  cell  next  con- 
stricts to  produce  two  daughter  cells,  each  containing 
a  group  of  daughter  chromosomes. 


4  * 


— v 


$• 

••*  — ^m 


•. 

_• 


•    A  •  w  •  • 

:^- 


•  «.••• 


%-A.     .«vV 


^.j^        ••r#*.<i;%< 

FIG.  19.  —  Fertilization  and  polar-body  formation  of  Nereis.  The 
four  smaller  figures  show  entrance  of  sperm.  The  extrusion  of  the  first 
polar  body  is  shown  in  lower  left-hand  figure  and  of  the  second  polar  body 
in  the  two  large  right-hand  figures.  The  last  three  also  show  the  formation 
of  the  sperm  asters,  which  is  the  beginning  of  the  first  cleavage  spindle  in 
the  egg.  (After  F.  R.  Lillie.) 

The  egg  is  also  a  cell,  and  in  its  earlier  stages  contains 
the  same  number  of  chromosomes  as  do  the  other  cells  of 
the  body;  but  after  two  peculiar  divisions  that  take 
place  at  maturation  the  number  of  the  chromosomes  is 
reduced  to  half. 


THE   MECHANISM   OF  SEX-DETERMINATION      37 

But  before  this  time  the  egg-cells  divide,  like  all  the 
other  cells  of  the  body.  In  this  way  a  large  number 
of  eggs  is  produced.  After  a  time  they  cease  to  divide 
and  begin  to  grow  larger,  laying  up  yolk  and  other 
materials.  At  this  time,  the  chromosomes  unite  in 
pairs,  so  that  their  number  seems  to  be  reduced  to  half 
the  original  number.  At  the  final  stage  in  the  matura- 
tion of  the  egg,  two  peculiar  divisions  take  place  that 
involve  the  formation  of  two  minute  cells  given  off  at 
one  pole  —  the  polar  bodies.  In  some  eggs,  as  in  the 
sea  urchin,  the  polar  bodies  are  given  off  while  the  egg 
is  still  in  the  ovary  and  before  fertilization;  in  other 
eggs,  as  in  the  frog,  one  polar  body  is  given  off  before 
fertilization,  the  other  after  the  sperm  has  entered ; 
and  in  other  eggs,  as  in  nereis  (Fig.  19),  both  polar 
bodies  are  given  off  after  fertilization. 

The  formation  of  the  polar  bodies  is  a  true  cell- 
division,  but  one  that  is  unique  in  two  respects. 
First,  one  of  the  cells  is  extremely  small,  as  seen  in 
Fig.  19.  The  smallness  is  due  to  the  minute  amount 
of  protoplasm  that  it  contains.  Second,  the  number  of 
chromosomes  at  each  division  is  the  half  or  "haploid  " 
number.  There  is  much  evidence  to  show  that  at  one 
or  at  the  other  of  these  two  divisions  the  two  chromo- 
somes that  had  earlier  united  are  separated,  and  in  this 
respect  this  division  differs  from  all  other  cell-divisions. 
In  consequence,  the  egg  nucleus,  that  re-forms  after  the 
second  polar  body  has  been  produced,  contains  only 
half  the  actual  number  of  chromosomes  characteristic 
of  all  the  other  cells  of  the  female. 

In  the  formation  of  the  spermatozoa  a  process  takes 
place  almost  identical  with  the  process  just  described 


38 


HEREDITY  AND  SEX 


for  the  female  (Fig.  20).  In  their  earlier  history  the 
germ-cells  of  the  male  divide  with  the  full  number  of 
chromosomes  characteristic  of  the  male,  which  may  be 
one  less  chromosome  than  in  the  female.  The  early 


FIG.  20.  —  A-B,  somatic  cell  division  with  four  chromosomes.  C-H, 
the  two  maturation  divisions  to  produce  the  four  cells  (H)  that  become 
spermatozoa.  (After  Wilson.) 

germ-cells  then  cease  to  divide  for  a  time,  and  begin 
to  grow,  laying  up  yolk  and  other  materials.  At  this 
time  the  chromosomes  unite  in  pairs,  so  that  the  num- 
ber appears  to  be  reduced  to  half.  Later  two  divisions 
occur  (Fig.  20,  D-H),  in  one  of  which  the  united  chro- 
mosomes separate.  The  male  germ-cells  differ,  how- 


THE  MECHANISM   OF  SEX-DETERMINATION      39 

ever,  from  the  female,  in  that  at  each  of  these  two  di- 
visions the  cells  are  equal  in  size.  Thus  four  sperm- 
cells  are  produced  from  each  original  cell,  all  four  pro- 
duce tails,  and  become  spermatozoa* 

At  the  time  of  fertilization,  when  the  spermatozoon 
touches  the  surface  of  the  egg,  the  egg  pushes  out  a  cone 
of  protoplasm  at  the  point  of  contact  (Fig.  19),  and, 
lending  a  helping  hand,  as  it  were,  to  the  sperm,  draws 
it  into  the  egg.  The  projecting  cone  of  protoplasm 
is  called  the  fertilization  cone.  In  a  few  minutes  the 
head  of  the  sperm  has  entered.  Its  tail  is  often  left 
outside.  The  head  absorbs  fluid  from  the  egg  and 
becomes  the  sperm  nucleus,  which  passes  towards  the 
center  of  the  egg.  Here  it  comes  to  lie  by  the  side  of  the 
egg  nucleus,  and  the  two  fuse.  The  walls  of  the  com- 
bined nuclei  dissolve  away  and  the  chromosomes  appear. 
Half  of  these  are  derived  from  the  father  through 
the  nucleus  of  the  sperm,  and  half  from  the  mother 
through  the  egg  nucleus.  If  we  count  the  paternal 
chromosomes,  there  are  half  as  many  of  them  as  there 
are  chromosomes  in  each  cell  of  the  body  of  the  fattier. 
Presently  I  shall  point  out  'that  this  statement  is  not 
always  true,  and  on  this  little  fact,  thatrit  is  not  quite 
true,  hangs  the  whole  story  of  sex-determination. 

What  is  the  meaning  of  these  curious  changes  that 
have  taken  place  in  the  egg  and  sperm  ?  Why  has  the 
egg,  before  developing,  twice  thrown  away  its  most 
valuable  heritage  —  its  chromatin  material  ?  We  do 
not  know  with  certainty,  but  one  consequence  at  least 
stands  out  clearly !  Before  the  egg  gave  off  its  polar 
bodies  it  had  the  full,  or  diploid,  number  of  chromo- 
somes. After  this  event  it  has  only  half  as  many.  A 


40  HEREDITY  AND   SEX 

similar  reduction  occurs  in  the  sperm,  excepting  that  no 
chromatin  is  lost,  but  is  redistributed  amongst  four 
spermatozoa.  Egg  and  sperm-nucleus  each  have  in 
consequence  the  haploid  or  half  number.  By  combin- 
ing they  bring  up  the  number  to  that  characteristic  of 
the  species. 

The  history  of  the  germ-cells,  that  we  have  just 
traced,  is  the  background  of  our  knowledge  of  the  pro- 
cess of  heredity  in  so  far  as  observable  changes  in  the 
germ-cells  have  been  made  out.  We  owe  to  Weismann 
more  than  to  any  other  biologist  the  realization  of 
the  importance  of  these  changes.  It  is  true  that 
Weismann  contributed  only  a  part  of  the  actual  facts 
on  which  the  interpretation  rests.  Many  workers, 
and  a  few  leaders,  have  laboriously  made  out  the  com- 
plete account.  But  Weismann,  by  pointing  out  the 
supreme  importance  of  the  changes  that  take  place  at 
this  time,  has  furnished  a  stimulus  that  has  acted  like 
yeast  in  the  minds  of  less  imaginative  workers. 

We  are  now  in  a  position  to  apply  this  knowledge  to 
the  interpretation  of  the  mechanism  by  means  of  which 
sex  is  determined. 

THE    CYTOLOGICAL   EVIDENCE 

If  we  study  by  means  of  modern  histological  methods 
the  body  cells  of  the  male  of  the  insect,  Protenor 
belfragei,  we  find,  when  each  cell  is  about  to  divide, 
that  a  group  of  chromosomes  appears  like  that  shown 
in  Fig.  21,  A.  There  are  twelve  ordinary  oval  chromo- 
somes, and  one  much  larger  than  the  rest.  This  group 
of  chromosomes  is  characteristic  of  all  divisions  of  the 
cells  of  the  body,  regardless  of  whether  the  cells  belong 


*    \si 
^ 


THE  MECHANISM  OF  SEX-DETERMINATION      41 

to  muscle,  skin,  gland,  ganglion,  or  connective  tissue. 
The  early  germ-cells  of  the  male,  the  so-called  "sper- 
matogonia,  '  '  also  have  this  same  number.  It  is  not  until 
a  later  stage  in  their  development  that  a  remarkable 
change  takes  place  in  them.  When  this  change  occurs 
the  thread-like  chromosomes  unite  in  pairs.  This  is 
the  synapsis  stage  =>-the  word  means  to  fuse  together. 

It  is  the  most  difficult  stage  to  interpret  in  the  whole 
history  of  the  germ-cells.  In  a  few  forms  where  the 
changes  that  take  place  have  been  seen  to  best  advan- 
tage it  is  found  that  chromosomes  are  in  the  form  of 
long  threads  and  that  these  threads  unite  in  pairs  to 
make  thicker  threads.  When  the  process  is  completed, 
we  find  half  as  many  threads  as  there  were  before.  This 
statement  is  not  quite  true.  In  the  case  of  the 
male  protenor,  for  instance,  there  are  twelve  ordinary 
chromosomes  and  one  large  one*  The  twelve  unite  in 
pairs  at  synapsis,  so  that  there  are  six  double  chromo- 
somes, but  the  large  one  has  no  mate  (Fig.  21,  B). 
When  the  others  have  united  in  synapsis,  it  has  taken 
no  Par^  in  the  process,  hence  the  reduced  number,  of 
chromosomes  in  the  male  is  seven  —  the  seventh  is 
the  sex  chromosome. 

Two  divisions  now  follow  each  other  in  rapid  succes- 
sion  (Fig.  21,  C,  D).  In  the  first  division  (C)  each 
chromosome  divides  —  seven  go  to  one  pole  and  seven 
to  the  other  pole.  Two  cells,  the  primary  spermato- 
cytes,  are  produced.  Without  resting^  another  divi- 
sion takes  place  (D)  irTeach  of  these  two  cells.  It  is 
the  second  spermatocyte  division.  Each  of  the  six 
ordinary  chromosomes  divides,  but  the  large  sex  chro- 
mosome does  not  divide,  and,  lagging  behind  the  others, 


42 


HEREDITY  AND  SEX 


as  shown  in  the  figure  (Z>),  it  passes  to  one  pole.  Each 
secondary  spermatocyte  produces,  therefore,  two  cells  — 
one  with  six,  the  other  with  seven  chromosomes.  These 
cells  become  spermatozoa  (EEf),  the  ones  with  seven 
chromosomes  are  the  female-producing  spermatozoa,  the 
ones  with  six  chromosomes  are  the  male-producing 

Prote/nor  3 


•  • 


D* 


FIG.  21. 


spermatozoa.     These  two  classes  of  spermatozoa  are 
present  in  equal  numbers. 

If  we  study  the  body  cells  of  the  female  protenor,  we 
find  fourteen  chromosomes  (Fig.  22,  A).  Twelve  of 
these  are  the  ordinary  chromosomes,  and  two,  larger 
than  the  rest,  are  the  sex  chromosomes.  At  the  synap- 
sis  stage  all  of  the  chromosomes  uni^e  in  pairs,  including 
the  two  sex  chromosomes.  When  the  pracess  ^.finished, 
there  are  seven  double  chromosomes  (Fife/.  2W,  3). 


THE  MECHANISM  OF  SEX-DETERMINATION      43 

When  the  egg  sends  off  its  two  polar  bodies,  the  chro- 
mosomes divide  or  separate.  At  the  first  division  seven 
chromosomes  pass  out  (C),  and  seven  remain  in  the 
egg.  At  the  next  division  the  seven  chromosomes  in 
the  egg  divide  again,  seven  pass  out  and  seven  remain 

Profenor  • 


in  the  egg  (D).  Of  these  seven,  one  chromosome, 
recognizable  by  its  large  size,  is  the  sex  chromosome. 

All  the  eggs  are  alike  (E) .  There  is  only  one  kind  of 
egg,  but  there  are  two  kinds  of  sperm.  Any  egg  that 
is  fertilized  by  a  sperm  carrying  six  chromosomes  pro- 
duces an  individual  with  thirteen  chromosomes.  This 
individual  is  a  male. 

Any  egg  that  is  fertilized  by  a  sperm  carrying  seven 


44 


HEREDITY  AND  SEX 


chromosomes   produces   an   individual   with   fourteen 
chromosomes.     This  individual  is  a  female. 

In  another  species  of  insect,  Lygaeus  bicrucis,  the  male 
differs   from   the   female,    not   in   having   a   different 


FIG.  23. 


number  of  chromosomes  as  in  protenor,  but  by  the 
occurrence  of  a  pair  of  different-sized  chromosomes. 

The  body  cells  of  the  male  have  twelve  ordinary 
chromosomes  and  two  sex  chromosomes  —  one  larger, 
X,  than  the  other,  Y  (Fig.  23,  A). 

After  synapsis  there  are  six  double  chromosomes  >and 
the  two  sex  chromosomes,  called  X  and  Y  (Fig.  23,  D). 


THE  MECHANISM  OF  SEX-DETERMINATION      45 

At  the  first  spermatocyte  division  all  the  chromosomes 
divide  (C).  The  two  resulting  cells  have  eight  chro- 
mosomes, including  X  and  Y.  At  the  second  division 
(D)  the  double  chromosomes  again  divide,  but  X  and  Y 
do  not  divide.  They  approach  and  touch  each  other, 
and  are  carried  into  the  spindle,  where  they  separate 
from  each  other  when  the  other"  ordinary  chromosomes 


/ 


divide.  Consequently  there  are  formed  two  kinds  of 
spermatozoa  —  one  containing  X  and  the  other  Y 
(Fig.  23,  E). 

In  the  body  cells  and  early  germ-tract  of  the  female. 
of   lygseus    (Fig.    24,   A),    there   are   twelve   ordinary 
chromosomes  and  two  sex  chromosomes,  K  and  X. 
After  reduction  there  are  seven  double  chromosomes, 
the  two  X's  having  united  when  the  other  chromosomes 


46  HEREDITY  AND  SEX 

united  (B).  Two  divisions  take  place  (C,  D),  when  the 
two  polar  bodies  are  formed,  leaving  seven  chromosomes 
in  the  egg  (E) .  Each  egg  contains  as  a  result  only  one 
X  chromosome* 

Any  egg  of  lygseus  fertilized  by  a  sperm  carrying  an 
X  chromosome  produces  a  female  that  contains  two 


&». 

tar 


FIG.  25. 


X's  or  XX.  Any  egg  fertilized  by  a  sperm  containing 
a  F  chromosome  produces  a  male  that  contains  one 
X  and  one  F,  or  XF. 

Another  insect,  Oncopeltus  fasciatus,  represents  a 
third  type  in  which  the  chromosome  groups  in  the 
male  and  in  the  female  are  numerically  alike  and  alike 
as  to  visible  size  relations. 


THE   MECHANISM  OF  SEX-DETERMINATION      47 

In  the  body  cells  of  the  male  there  are  sixteen  chro- 
mosomes (Fig.  25,  A).  After  reduction  there  are  nine 
chromosomes  —  seven  in  a  ring  and  two  in  the  middle 
(B).  The  seven  are  the  fused  pairs  or  double  chro- 
mosomes ;  the  two  in  the  middle  are  the  sex  chromo- 
somes that  have  not  fused. 


The  evidence  for  this  interpretation  is  circumstan- 
tial but  sufficient. 

At  the  first  reduction  division  all  nine  chromosomes 
divide  (C).  Just  before  the  second  division  the  two 
central  chromosomes  come  together  and  remain  in 
contact  (DDf).  All  the  double  chromosomes  then 
divide,  while  the  two  sex  chromosomes  simply  sepa- 
rate from  each  other,  so  that  there  are  eight  chromo- 
somes at  each  pole  (DE). 


48 


HEREDITY  AND   SEX 


In  this  case  all  of  the  spermatozoa  (EE')  contain 
eight  chromosomes.  There  is  no  visible  difference 
between  them.  Nevertheless,  there  is  reason  for  be- 
lieving that  here  also  there  are  two  kinds  of  sperm. 
The  principal  reason  is  that  there  are  all  connecting 
stages  between  forms  in  which  there  is  an  unequal  pair, 


FIG.  27. 


as  in  lygseus,  and  forms  with  an  equal  pair,  as  in  oncopel- 
tus.  Another  reason  is  that  the  two  sex  chromosomes 
behave  during  the  synapsis  stages  as  do  the  X  Y  chromo- 
somes in  related  species.  Moreover,  the  experimental 
evidence,  of  which  I  shall  speak  later,  leads  us  to  con- 
clude that  the  determination  of  sex  is  not  due  only  to 


THE  MECHANISM   OF  SEX-DETERMINATION    49 

a  difference  in  size  of  X  and  Y .  The  sex  chromosomes 
must  carry  a  host  of  factors  other  than  those  that  de- 
termine sex.  Consequently  it  is  not  surprising  that  in 
many  species  the  sex  chromosomes  appear  equal  or 
nearly  equal  in  size.  It  is  a  fortunate  circumstance  for 
us  that  in  some  species  there  is  &  difference  in  size  or 


an  unpaired  sex  chromosome ;  for,  in  consequence,  we 
are  able  to  trace  the  history  of  each  kind  of  sperm  in 
these  cases ;  but  it  is  not  essential  to  the  theory  that 
X  and  Y,  when  present,  should  be  visibly  different. 

In  the  female  of  oncopeltus  sixteen  chromosomes 
occur  as  in  the  male  (Fig.  26,  A).  The  reduced  number 
is  eight  double  chromosomes  (B).  At  one  of  the  two 
polar  divisions  eight  chromosomes  pass  out,  and  eight 
remain  in  the  egg  (C).  At  the  second  division  also 
eight  pass  out,  and  eight  remain  in  the  egg  (D). 


50  HEREDITY  AND   SEX 

I  shall  pass  now  to  a  fourth  condition  that  has  only 
recently  come  to  light.  It  is  best  shown  in  some  of  the 
nematode  worms,  for  example,  in  the  ascaris  of  the 
horse.  Here  the  sex  chromosomes  are  generally  at- 
tached to  other  chromosomes.  In  this  case,  as  shown 
by  the  diagram  (Fig.  27,  A),  there  is  in  the  male  a  single 
X  attached  to  one  of  the  other  chromosomes.  At  the 
first  spermatocyte  division  it  does  not  divide  (C), 
but  passes  over  bodily  to  one  pole,  so  that  two  kinds 
of  cells  are  produced.  At  the  second  spermatocyte 
division  it  divides,  in  the  cell  that  contains  it,  so  that 
each  daughter  cell  gets  one  X  (D).  Two  classes  of 
sperm  result,  two  with  X  (E),  two  without  (Ef). 

In  the  female  there  are  two  X'Sj  each  attached  to  a 
chromosome  (Fig.  28).  After  the  polar  bodies  are 
given  off,  one  X  only  is  left  in  each  egg  (C,  Z),  E).  Sex 
is  determined  here  in  the  same  way  as  in  the  insects, 
described  above,  for  there  are  two  classes  of  sperm  and 
but  one  class  of  eggs. 

The  discovery  of  the  sex  chromosome  and  its  rela- 
tion to  sex  is  due  to  several  investigators.  In  1891 
Henking  first  described  this  body,  and  its  unequal  distri- 
bution, but  was  uncertain  even  as  to  its  relation  to  the 
chromosomes.  Paulmier  (1899),  Montgomery  (1901), 
Sinety  (1901),  gave  a  correct  description  of  its  behavior 
in  spermatogenesis.  McClung  (1902)  confirmed  these 
discoveries,  and  suggested  that  the  accessory,  or  odd 
chromosome,  as  it  was  then  called,  had  some  relation 
to  sex,  because  of  its  unequal  distribution  in  the 
sperms.  He  inferred  that  the  male  should  have  one 
more  chromosome  than  the  female,  but  he  gave  no  evi- 
dence in  support  of  this  suggestion,  which  as  we  have 


THE  MECHANISM   OF  SEX-DETERMINATION      51 

seen  is  the  reverse  of  the  actual  conditions.  Stevens 
(1905)  made  out  the  relations  of  the  XY  pair  of  chro- 
mosomes to  sex  and  Wilson  in  the  same  year  (1905) 
the  correct  relation  of  the  accessory  chromosome  to  sex. 
The  results  described  above  for  the  insects  are  for  the 
most  part  from  Wilson's  studies  on  the  chromosomes ; 
those  for  ascaris  from  the  recent  work  of  Sophia 
Frolowa,  which  confirms  in  the  main  the  work  of  Boveri, 
Gulick,  Boring,  and  Edwards. 

In  the  fruit  fly,  Drosophila  ampelophila,  it  appears 
from  the  recent  work  of  Metz,  that,  in  the  male,  there 
is  an  XY  pair  of  sex  chromosomes,  instead  of  only  an 
X,  as  Stevens  supposed.  The  female  has,  of  course, 
two  X's.  An  analysis  of  certain  experimental  evi- 
dence has  led  H.  J.  Muller  to  the  conclusion  that  prob- 
ably the  Y  chromosome  carries  no  factors  that  influence 
development.  If  this  proves  true,  we  can  better  under- 
stand how  it  might  be  completely  lost  in  certain  types. 

The  whole  history  of  the  sex  chromosomes  of  ancyro- 
canthus,  a  nematode  worm,  is  strikingly  shown  in  a 
recent  paper  by  Carl  Mulsow  (Fig.  29  and  29a,  A). 
This  is  a  typical  case  in  which  the  male  has  one  less 
chromosome  than  the  female,  as  in  protenor.  The 
case  is  striking  because  the  chromosomes  can  be  seen 
and  counted  in  the  living  spermatozoa.  Some  sperm 
have  six,  some  have  five  chromosomes.  The  sperm- 
nucleus  can  be  identified  in  the  egg  after  fertilization 
because  it  lies  nearer  the  pole  opposite  to  the  polar 
bodies.  The  entering  sperm  nuclei  show  in  half  of 
the  fertilized  eggs  six  chromosomes  and  in  the  other 
half  five  chromosomes. 

An  interesting  confirmation  of  these  conclusions  in 


52  HEREDITY  AND   SEX 

regard  to  the  relation  between  sex  and  the  sex  chromo- 
somes was  found  in  another  direction.  It  has  long  been 
known  that  the  fertilized  eggs  of  aphids  or  plant  lice 
produce  only  females.  The  same  thing  happens  in 
near  relatives  of  the  plant  lice,  the  phylloxerans. 


-y- 


2 


<»        t 

,<!t  '•'". 


FIG.  29.  —  1  and  2  are  spermatogonia  ;  3,  growth  period  ;  4-7,  prophases  ; 
8,  equatorial  plate  of  first  division,  9-10  ;  11,  spermatocytes  of  second  order ; 
12-13,  division  of  same;  14-16,  the  four  cells  or  spermatids  that  come 
from  the  same  original  cell,  two  with  5,  two  with  6  chromosomes;  17, 
spermatids;  18,  mature  sperm;  19,  living  sperm.  (After  Mulsow.) 

In  these  insects  a  study  of  the  chromosomes  shows 
that  the  male  has  one  less  chromosome  than  the  female. 
At  the  first  maturation  division  in  the  male  (Fig.  30), 
all  the  chromosomes  divide  except  one,  the  X  chromo- 
some, and  this  passes  to  one  cell  only.  This  cell  is 
also  larger  than  the  sister  cell.  The  small  cell  lacking 
the  X  degenerates,  and  does  not  produce  spermato- 


THE  MECHANISM   OF  SEX-DETERM I  NATION      53 

zoa.  The  large  cell  divides  again,  all  of  the  chromo- 
somes dividing.  Two  functional  spermatozoa  are 
produced,  each  carrying  one  sex  chromosome.  These 
spermatozoa  correspond  to  the  female-producing  sper- 
matozoa of  other  insects. 

In  the  sexual  female  there  is  an  even  number  of  chro- 


• 


Oil 


•) 


FIG.  29a.  —  20  and  21,  oogonia  (equatorial  plate);  22,  growth  period; 
23,  before  fertilization:  24-25,  entrance  of  sperm;  26-31,  prophases  of 
first  division ;  32—33,  formation  of  first  polar  body ;  34—36,  extrusion  of 
same  and  formation  of  second  polar  body  ;  37,  two  pronuclei ;  38-41,  union 
of  pronuclei ;  42-45,  cleavage.  (After  Mulsow.) 

mosomes  —  one  more  than  in  the  male.  They  unite 
in  pairs.  When  the  two  polar  bodies  of  the  sexual 
egg  are  formed,  all  the  chromosomes  divide  twice,  so 
that  each  egg  is  left  with  one  sex  chromosome. 

It  is  now  evident  why  only  females  are  produced 
after  fertilization.  The  female-producing  sperm  alone 
is  functional. 


54 


HEREDITY  AND   SEX 


FIG.  30.  —  Diagram  of  chromosomes  in  Phylloxera,  carycecaulis.  Top 
line,  somatic  cell  of  female  with  6  chromosomes  and  somatic  cell  of  male 
with  5  chromosomes.  Second  line,  stages  in  first  spermatocyte  division 
producing  a  rudimentary  cell  (below)  with  two  chromosomes.  Third  line, 
second  spermatocyte  division  into  two  equal  cells.  Fourth  line,  sexual 
egg  (3  chromosomes)  and  two  polar  bodies  ;  and  two  functional,  female- 
producing  sperm  with  three  chromosomes  each. 


THE  MECHANISM   OF  SEX-DETERMINATION      55 
THE    EXPERIMENTAL   EVIDENCE 

The  experimental  evidence,  indicating  that  there  is 
an  internal  mechanism  for  sex  determination,  is  derived 
from  two  sources  —  from  experimental  embryology,  and 
from  a  study  of  the  heredity  of  sex-linked  characters. 

The  evidence  from  embryology  shows  that  the  chro- 
mosomes are  the  bearers  of  materials  essential  for  the 
production  of  characters.  The  evidence  from  hered- 
ity shows  that  certain  characters  follow^  the  sex 
chromosomes. 

It  has  long  been  taught  that  the  hereditary  factors 
are  carried  by  the  nucleus.  The  evidence  for  this  was 
found  in  fertilization.  When  the  spermatozoon  enters 
the  egg,  it  carries  in,  as  a  rule,  only  the  head  of  the  sper- 
matozoon, which  consists  almost  entirely  of  the  nucleus 
of  the  original  cell  from  which  it  comes.  Since  the 
male  transmits  his  characters  equally  with  the  female, 
it  follows  that  the  nucleus  is  the  source  of  this 
inheritance. 

The  argument  has  not  been  regarded  as  entirely 
conclusive,  because  the  sperm  may  also  bring  in  some 
of  the  protoplasm  of  the  original  cell — at  least  that  part 
lying  immediately  around  the  nucleus.  In  addition  a 
small  body  lying  at  the  base  of  the  sperm  head  seems 
also  to  be  brought  in  by  the  male,  and  according  to 
some  observers  it  becomes  the  center  about  which  the 
entire  division  system  or  karyokinetic  spindle  develops. 

The  most  convincing  evidence  that  the  chromosomes 
are  the  most  important  elements  in  heredity  is  found  in 
some  experimental  work,  especially  that  of  Boveri, 
Baltzer,  and  Herbst.  Under  certain  circumstances  in 


56 


HEREDITY  AND   SEX 


the  sea-urchin  two  spermatozoa  may  enter  a  single 
egg.  They  both  unite  with  the  egg  nucleus  (Fig.  31). 
Each  brings  in  18  chromosomes.  The  egg  contributes 
18  chromosomes.  There  are  in  all  54,  instead  of  36 
chromosomes,  as  in  normal  fertilization. 


FIG.  31.  —  Dispermy  and  its  effects  in  egg  of  sea  urchin.     (After 
Boveri.) 

Around  these  chromosomes  a  double  system  of 
threads  develops  with  four  poles.  The  chromosomes 
become  unequally  distributed  on  the  four  spindles  that 
develop.  Each  chromosome  then  divides,  and  half  of 
each  goes  to  the  nearest  pole.  To  some  of  the  poles 
many  chromosomes  may  pass,  to  other  poles  fewer. 


THE  MECHANISM  OF  SEX-DETERMI NATION      57 

In  order  to  simplify  the  case  let  us  imagine  that  each 
sperm  has  only  four  chromosomes  and  the  egg  nucleus 
only  four.  Let  us  represent  these  by  the  letters  as 
shown  in  Fig.  32.  Any  one  of  the  four  cells  that  is 


FIG.  32.  —  Diagram  illustrating  the  irregular  distribution  of  the  chro- 
mosomes in  dispermic  eggs  in  an  imaginary  case  with  only  four  kinds  of 
chromosomes,  a,  b,  c,  d.  There  are  here  three  sets  of  each  of  these  in 
each  egg.  The  stippled  cells  are  those  that  fail  to  receive  one  of  each 
kind  of  chromosome.  (After  Boveri.) 

produced  at  the  first  division  of  these  dispermic  eggs 
may  contain  a  full  complement  of  the  chromosomes, 
or  only  some  of  them.  The  possibilities  for  four 
chromosomes  are  shown  in  the  diagram.  Any  cell 
that  does  not  contain  at  least  these  four  chromosomes 
is  shaded.  One  case  is  present  in  which  all  the  four 


58  HEREDITY  AND   SEX 

cells  contain  a  complete  assortment.  If  normal  devel- 
opment depends  on  an  embryo  containing  in  every  cell 
at  least  one  of  each  kind  of  chromosome,  then  in 
our  simple  case  only  one  group  of  four  cells  has  this 
possibility. 

Boveri  found  that  such  dispermic  eggs  produce 
normal  embryos  very  rarely.  He  calculated  what  the 
chance  would  be  when  three  times  18  chromosomes 
are  involved.  The  chance  for  normal  development 
is  probably  not  once  in  10,000  times.  He  isolated 
many  dispermic  eggs  and  found  that  only  one  in  1,500 
of  the  tetrad  type  developed  normally. 

Boveri  went  still  further  in  his  analysis  of  the  prob- 
lem. It  had  been  shown  for  normal  eggs  that  if  at 
the  two-celled  stage  the  cells  are  separated,  each  forms 
a  perfect  embryo.  This  is  also  true  for  each  of  the 
first  four  cells  of  the  normal  egg. 

Boveri  separated  the  four  cells  of  dispermic  eggs  and 
found  that  the  quadrants  not  infrequently  developed 
normally.  This  is  what  we  should  anticipate  if  those 
cells  can  develop  that  contain  one  of  each  kind  of 
chromosome. 

The  evidence  furnishes  strong  support  of  the  view 
that  the  chromosomes  are  different  from  each  other, 
and  that  one  of  each  kind  is  necessary  if  development 
is  to  take  place  normally. 

The  evidence  that  Baltzer  has  brought  forward  is 
also  derived  from  a  study  of  sea-urchin  eggs.  It  is 
possible  to  fertilize  the  eggs  of  one  species  with  sperm 
of  another  species.  The  hybridizing  is  greatly  helped 
by  the  addition  of  a  little  alkali  to  the  sea  water. 

Baltzer  made  combinations  between  four  species  of 


THE   MECHANISM   OF  SEX-DETERMIXATION      59 


sea-urchins.  We  may  take  one  cross  as  typical.  When 
eggs  of  strongylocentrotus  are  fertilized  with  sperm  of 
sphserechinus,  it  is  found  at  the  first  division  of  the  egg 
that,  while  some  of  the  chromosomes  divide  and  pass 
normally  to  the  two  poles,  other  chromosomes  remain 
in  place,  or  become  scattered  irregularly  between  the 
two  poles,  as  shown  in  Fig.  33.  When  the  division 


;• 


FIG.  33.  —  1  and  la,  chromosomes  in  the  normal  first  cleavage  spindle  of 
Sphaerechinus ;  2,  equatorial  plates  of  two-cell  stage  of  same ;  3-3a,  hybrid, 
Sphaerechinus  by  Strongylocentrotus,  spindle  at  two-cell  stage ;  4-4a,  same 
equatorial  plates ;  5-5a,  hybrid,  Strong,  by  Sphser.,  cleavage  spindle  in  telo- 
phase  ;  6,  next  stage  of  last ;  7,  same,  two-cell  stage ;  8,  same,  later ;  9,  same, 
four-cell  stage  ;  10,  same,  equatorial  plate  in  two-cell  stage  (12  chromosomes) ; 
11,  same,  from  later  stage,  2-4  chromosomes.  (After  Baltzer.) 

is  completed,  some  of  these  chromosomes  are  found 
outside  of  the  two  main  nuclei.  They  often  appear 
as  irregular  granules,  and  show  signs  of  degeneration. 
They  are  still  present  as  definite  masses  after  the  next 
division,  but  seem  to  take  no  further  part  in  the  de- 
velopment. 

Baltzer  has  attempted  to  count  the  number  of  chro- 
mosomes in  the  nuclei  of  these  hybrid  embryos.  The 


60  HEREDITY  AND   SEX 

number  is  found  to  be  about  twenty-one.  The  maternal 
egg  nucleus  contains  eighteen  chromosomes.  It  appears 
that  only  three  of  the  paternal  chromosomes  have 
succeeded  in  getting  into  the  regular  cycle  —  fifteen  of 
them  have  degenerated. 

Baltzer  thinks  that  the  egg  acts  injuriously  in  this 
case  on  the  chromosomes  of  foreign  origin,  especially 
on  the  fifteen  that  degenerate,  so  that  they  are  elim- 
inated from  the  normal  process. 

The  embryos  that  develop  from  these  eggs  are  often 
abnormal.  A  few  develop  as  far  as  the  pluteus  stage, 
when  a  skeleton  appears  that  is  very  characteristic  for 
each  species  of  sea-urchin.  The  plutei  of  these  hybrids 
~are  entirely  maternal.  This  means  that  they  are 
exactly  like  the  plutei  of  the  species  to  which  the 
mother  belongs. 

The  conclusion  is  obvious.  The  sperm  of  sphserechi- 
nus  has  started  the  process  of  development,  but  has 
produced  no  other  effect,  or  has  at  most  only  slightly 
affected  the  character  of  the  offspring.  It  is  reason- 
able to  suppose  that  this  is  because  of  the  elimination 
of  the  paternal  chromosomes,  although  the  evidence 
is  not  absolutely  convincing. 

Let  us  now  examine  the  reciprocal  cross.  When  the 
eggs  of  sphserechinus  are  fertilized  by  the  sperm  of 
strongylocentrotus,  the  division  of  the  egg  and  of  the 
chromosomes  is  entirely  normal.  All  the  chromosomes 
divide  and  pass  to  the  poles  of  the  spindle.  The  total 
number  (36)  must,  therefore,  exist  in  each  cell,  although 
in  this  case  they  were  not  actually  counted. 

The  pluteus  that  develops  has  peculiarities  of  both 
maternal  and  paternal  types.  It  is  hybrid  in  structure. 


THE  MECHANISM   OF  SEX-DETERMINATION      61 

Both  parents  have  contributed  to  its  formation.  It  is 
not  going  far  from  the  evidence  to  infer  that  the  hybrid 
character  is  due  to  both  sets  of  chromosomes  being 
present  in  all  of  the  cells. 


FIG.  34.  —  1.  The  chromosomes  of  the  egg  lie  in  the  equator  of  the 
spindle,  the  chromosomes  of  the  sperm  lie  at  one  side.  2.  A  later  stage, 
showing  all  the  paternal  chromosomes  passing  to  one  pole.  3  (to  the  right). 
A  later  stage,  a  condition  like  the  last.  There  is  also  a  supernumerary  sperm 
in  the  egg  (to  left,  in  another  section.)  4.  Same  condition  as  last.  5.  Plu- 
teus  larva  that  is  purely  maternal  on  one  side  and  hybrid  on  the  other. 
(After  Herbst.) 

The  evidence  that  Herbst  has  brought  forward  is  of 
a  somewhat  different  kind.  It  supplements  Baltzer's 
evidence  and  makes  more  probable  the  view  that  the 
chromosomes  are  essential  for  the  development  of  the 
characters  of  the  individual. 


62  HEREDITY  AND   SEX 

Herbst  put  the  eggs  of  sphaerechinus  into  sea  water 
to  which  a  little  valerianic  acid  had  been  added.  This 
is  one  of  the  many  methods  that  Loeb  has  discovered 
by  which  the  egg  may  be  induced  to  develop  parthe- 
nogenetically,  i.e.  without  the  intervention  of  the  sper- 
matozoon. After  five  minutes  the  eggs  were  removed  to 
pure  sea  water  and  the  sperm  of  another  species,  stron- 
gylocentrotus,  was  added.  The  sperm  penetrated  some 
of  the  eggs.  The  eggs  had  already  begun  to  undergo 
the  changes  that  lead  to  division  of  the  cell  —  the  sperm 
entered  ten  minutes  late.  The  egg  proceeded  to 
divide,  the  sperm  failed  to  keep  pace,  and  fell  behind. 
Consequently,  as  shown  in  Fig.  34,  the  paternal 
chromosomes  fail  to  reach  the  poles  when  the  nuclei 
are  re-formed  there.  The  paternal  chromosomes  form 
a  nucleus  of  their  own  which  comes  to  lie  in  one  of  the 
two  cells.  In  consequence  one  cell  has  a  nucleus  that 
contains  only  the  maternal  chromosomes ;  the  other 
cell  contains  two  nuclei,  one  maternal  and  the  other 
paternal.  In  later  development  the  paternal  nucleus 
becomes  incorporated  with  the  maternal  nucleus  of  its 
cell.  There  is  produced  an  embryo  which  is  maternal 
on  one  side  and  hybrid  on  the  other.  Herbst  found  in 
fact  that  half-and-half  plutei  were  not  rare  under  the 
conditions  of  his  experiment. 

This  evidence  is  almost  convincing,  I  think,  in 
favor  of  the  view  that  the  chromosomes  are  the  es- 
sential bearers  of  the  hereditary  qualities.  For  in 
this  case,  whether  the  protoplasm  of  the  embryo 
comes  from  the  egg  or  the  sperm  also,  the  fact  re- 
mains that  the  half  with  double  nuclei  is  hybrid. 
Even  if  the  spermatozoon  brought  in  some  proto- 


- 


THE   MECHANISM   OF  SEX-DETERMINATION      63 


plasm,  there  is  no  reason  to  suppose  that  it  would 
be  distributed  in  the  same  way  as  are  the  paternal 
chromosomes! 

EVIDENCE    FROM    SEX-LINKED    INHERITANCE 

The  experimental  evidence  based  on  sex-linked  in- 
heritance may  be  illustrated  by  the  following  examples. 

The  eyes  of  the  wild  fruit-fly,  Drosophila  ampe- 
lophila,  are  red.  In  my  cultures  a  male  appeared  that 
had  white  eyes.  He  was  mated  to  a  red-eyed  female. 
The  offspring  were  all  red-eyed  j~-  both  males  and 
females  (Fig.  35).  These  were  inbred  and  produced 
in  the  next  generation  red*-eyed  females,  red-eyed  males, 
and  white-eyed  males  (Fig.  35).  There  were  no  white- 
eyed  females.  The  white-eyed  grandfather  had  trans- 
mitted white  eyes  to  half  of  his  grandsons  but  to  none 
of  his  granddaughters.- 

Equally  important  are  the  numerical  proportions 
in  which  the  colors  appear  in  the  grandchildren.  There 
are  as  many  females  as  the  two  classes  of  males  taken 
together;  half  of  the  males  have  red  eyes  and  half 
have  white  eyes.  The  proportions  are  therefore  50  % 
red  females,  25  %  red  males,  25  %  white  males. 

Only  white-eyed  males  had  appeared  at  this  time. 
It  may  seem  that  the  eye  color  is  confined  to  the  male 
sex*  Hence  the  origin  of  the  term  sex-limited  inheri- 
tance for  cases  like  this.  But  white-eyed  females  may 
be  produced  easily.  If  certain  of  the  rted-eyed  grand- 
daughters are  bred  to  these  white-eyed  males,  both 
white-eyed  females  and  males,  and  red-eyed  females 
and  males,  appear  (Fig.  37).  The  white  eye  is  there- 
fore not  sex-limited  but  sex-linked^ 


64 


HEREDITY  AND   SEX 


With  these  white-eyed  females  it  is  possible  to  make 
the  reciprocal  cross  (Fig.  36).  A  white-eyed  female 
was  mated  to  a  red-eyed  male,.  All  of  the  daughters 
had  red  eyes  and  all  the  sons  had  white  eyes.  These 
were  then  inbred  and  gave  red-eyed  males  and  females,, 


xx 


FIG.  35.  —  Sex-linked  inheritance  of  white  and  red  eyes  in  Drosophila. 
Parents,  white-eyed  $  and  red-eyed  9  ;  Fi,  red-eyed  $  and  9  ;  F2  red- 
eyed  9  >  red-eyed  $  and  white-eyed  $  .  To  right  of  flies  the  history  of  the 
sex  chromosomes  XX  is  shown.  The  black  X  carries  the  factor  for  red 
eyes,  the  open  X  the  factor  for  white  eyes ;  the  circle  stands  for  no  X. 

and  white-eyed  males  and  females  in  equal  numbers 
(Fig.  36). 

The  heredity  of  this  eye  color  takes  place  with  the 
utmost  regularity,  and  the  results  show  that  in  some 
way  the  mechanism  that  is  involved  is  closely  bound 
up  with  the  mechanism  that  produces  sex. 


THE  MECHANISM   OP  SEX-DETERMIXATlOX      65 

Other  combinations -give  results  that  are  predictable 
from  those  just  cited.  For  instance,  if  the  FI  red-eyed 
female  from  either  of  the  preceding  crosses  is  mated  to 
a  white-eyed  male,  she  produces  red-eyed  males  and 
females,  and  white-eyed  males  and  females,  as  shown  in 


w 

/v 


FIG.  36.  —  Reciprocal   cross  of  Fig.   35,     Parents,  white-eyed    9    and 

red-eyed    $,    (criss-cross    inheritance).      FI,    red-eyed  9.   white-eyed    $. 

Y-,  red-eyed  9  and  $  ;  white-eyed  9  and  $ .  To  right,  sex  chromo- 
somes (as  in  Fig.  35). 

Fig.  37  (upper  two  lines).  If  the  FI  red-eyed  male 
from  the  first  cross  (Fig.  35)  is  bred  to  a  white-eyed 
female,  he  will  produce  red-eyed  daughters  and  white- 
eyed  sons.  Fig.  37  (lower  two  lines). 

The  same  relations  may  next  be  illustrated  by  an- 
other sex-linked  character. 


66 


HEREDITY  AND   SEX 


A  male  with  short  or  miniature  wings  appeared  in 
my  cultures  (Fig.  38).  Mated  to  long- winged  females 
only  long-winged  offspring  were  produced.  When 
these  were  mated  to  each  other,  there  were  produced 


x 


FIG.  37.  —  Upper  series,  back  cross  of  FI    9   to  white   $  .     Lower  series 
back  cross  of  FI  red-eyed  $  to  white  9  • 

long-winged  females  (50%),  long- winged  males  (25%) 
and  miniature-winged  males  (25%). 

It  is  possible  to  produce,  in  the  way  described  for 
eye  color,  miniature-winged  females. 

When  such  miniature-winged  females  are  mated  to 
long- winged  males,  all  the  daughters  have  long  wings, 
and  all  the  sons  have  miniature  wings  (Fig.  39).  If 


THE  MECHANISM   OR  SEX-DETERMINATION      67 

these  are  now  mated,  they  produce,  in  equal  numbers, 
long-winged  males  and  females  and  miniature-winged 
males  and  females. 

The  same  relations  may  again  be  illustrated  by  body 
color. 


xx 


XXXIX® 


FIG.  38.  —  Sex-linked  inheritance  of  short  ("miniature")  and  long  wings 
in  Drosophila.  Parents,  short -winged  $ ,  long-winged  9  •  Fi  long-winged 
$  and  9  •  F2  long-winged  9  and  $  and  short-winged  $ .  Sex  chromo- 
somes to  right.  Open  X  carries  short  wings. 

A  male  appeared  with  yellow  wings  and  body.  Mated 
to  wild  gray  females  he  produced  gray  males  and 
females.  These  mated  to  each  other  gave  gray  females 
(50%),  gray  males  (25%),  and  yellow  males  (25%). 

As  before,  yellow  females  were  made  up.  Mated  to 
gray  males  they  gave  gray  females  and  yellow  males. 


68 


HEREDITY  AND   SEX 


These  inbred  gave  gray  males  and  females  and  yellow 
males  and  females,  in  equal  numbers. 

These  cases  serve  to  illustrate  the  regularity  of  this 
type  of  inheritance  and  its  relation  to  sex.  In  the  fruit 
fly  we  have  found  as  many  as  twenty-five  sex-linked 


FIG.  39.  —  Reciprocal  cross  of  Fig.  38.  Parents,  long-winged  $  and 
short-winged  9 .  Fi  long-winged  9 .  short-winged  $ .  F2  long-winged 
9  and  $  ,  short-winged  9  and  $  •  Sex  chromosomes  as  in  last. 

factors.  There  are  other  kinds  of  inheritance  found  in 
these  flies,  and  at  another  time  I  shall  speak  of  some 
of  these ;  but  the  group  of  sex-linked  factors  is  of  special 
importance  because  through  them  we  get  an  insight 
into  the  heredity  of  sex. 

In  the  next  chapter,  when  we  take  up  in  detail  Men- 
delian  heredity,  I  shall  try  to  go  further  into  the  ex- 


THE   MECHANISM  OF.  SEX-DETERMINATION      69 

planation  of  these  facts^  For  the  present  it  will  suffice 
to  point  out  that  the  cases  just  described,  and  all  like 
them,  may  be  accounted  for  by  means  of  a  very  simple 
hypothesis.  We  have  traced  the  history  of  the  X 
chromosomes.  If  the  factors  that  produce  white  eyes, 
short  (miniature)  wings,  and  yellow  body  color  are 
carried  by  the  X  chromosomes,  we  can  account  for 
these  results  that  seem  at  first  sight  so  extraordinary. 
The  history  of  the  sex  chromosomes  is  accurately  known. 
Their  distribution  in  the  two  sexes  is  not  a  matter  of 
conjecture  but  a  fact.  Our  hypothesis  rests  therefore 
on  a  stable  foundation.. 

At  the  risk  of  confusion  I  feel  bound  to  present  here 
another  type  of  sex-linked  inheritance*  In  principle 
it  is  like  the  last,  but  the  actual  mechanism,  as  we  shall 
see,  is  somewhat  different.  Again  I  shall  make  use 
of  an  illustration,  If  a  black  Langshan  hen  is  mated 
to  a  barred  Plymouth  Rock  cock,  all  the  offspring  will 
be  barred  (Fig.  40) .  If  these  are  inbred,  there  are  pro- 
duced barred  females  and  males,  and  black  females. 
The  numerical  proportion  is  50  per  cent  barred  males,  25 
per  cent  barred  females,  and  25  per  cent  black  females. 

The  black  hen  has  transmitted  her  character  to  half 
of  her  granddaughters  and  to  none  of  her  grandsons. 
The  resemblance  to  the  case  of  the  red-eyed,  white- 
eyed  flies  is  obvious,  but  here  black  appears  as  a  sex- 
linked  character  in  the  females. 

The  converse  cross  is  also  suggestive.  When  a 
barred  hen  is  mated  to  a  black  cock,  all  the  daughters 
are  black  and  all  the  sons  are  barred  (Fig.  41).  When 
these  are  inbred,  there  are  produced  black  males  and 
females  and  barred  males  and  females  in  equal  num- 


70 


HEREDITY  AND   SEX 


bers.  Again,  the  resemblance  of  the  reciprocal  cross 
to  one  of  the  combinations  for  eye  color  is  apparent. 
In  fact,  this  case  can  be  explained  on  the  same  prin- 
ciple as  that  used  for  the  flies,  except  that  in  birds  it  is 


FIG.  40.  — Sex-linked  inheritance  in  fowls.  Upper  line  black  Laugshan 
hen  and  barred  Plymouth  Rock  cock.  Second  line,  Flt  barred  cock  and 
hen.  Third  line,  F2,  three  barred  (cock,  hen,  cock)  and  one  black  (hen). 
(Cuts  from  "  Reliable  Poultry  Journal.")  F\  and  F2  for  color  only. 


THE  MECHANISM   OE  SEX-DETERMINATION      71 

the  female  that  produces  two  kinds  of  eggs;  she  is 
heterozygous  for  a  sex  factor  while  the  male  produces 
only  one  kind  of  spermatozoon. 


FIG.  41.  —  Reciprocal  cross  of  Fig.  40.  Upper  line,  black  cock  and 
barred  hen.  Second  line,  Fi,  barred  cock  and  black  hen.  Third  line,  F2, 
barred  hen  and  cock,  black  cock  and  hen.  (Cuts  from  "  Reliable  Poultry 
Journal.")  FI  and  FZ  correct  for  color  pattern  only. 


72  HEREDITY  AND   SEX 

We  lack  here  the  certain  evidence  from  cytology  that 
we  have  in  the  case  of  the  insects.  Indeed,  there  is 
some  cytological  evidence  to  show  that  the  male  bird 
is  heterozygous  for  the  sex  chromosome.  But  the 
evidence  does  not  seem  to  me  well  established ;  while 
the  experimental  evidence  is  definite  and  has  been 
independently  obtained  by  Bateson,  Pearl,  Sturtevant, 
Davenport,  Goodale  and  myself.  However  this  may  be, 
the  results  show  very  clearly  that  here  also  sex  is  con- 
nected with  an  internal  mechanism  that  is  concerned 
with  other  characters  also.  This  is  the  mechanism  of 
Mendelian  heredity.  Whether  the  chromosomes  suffice 
or  do  not  suffice  to  explain  Mendelian  heredity,  the 
fact  remains  that  sex  follows  the  same  route  taken  by 
characters  that  are  recognized  as  Mendelian. 

To  sum  up :  The  facts  that  we  have  considered 
furnish,  I  believe,  demonstrative  evidence  in  favor 
of  the  view  that  sex  is  regulated  by  an  internal  mech- 
anism. The  mechanism  appears,  moreover,  to  be  the 
same  mechanism  that  regulates  the  distribution  of  cer- 
tain characters  that  follow  Mendel's  law  of  inheritance. 


CHAPTER   III 

THE  MENDELIAN  PRINCIPLES  OF  HEREDITY  AND 
THEIR  BEARING  ON  SEX 

THE  modern  study  of  heredity  dates  from  the  year 
1865,  when  Gregor  Mendel  made  his  famous  discoveries 
in  the  garden  of  the  monastery  of  Briinn.  For  35 
years  his  paper,  embodying  the  splendid  results  of  his 
work,  remained  unnoticed.  It  suffered  the  fate  that 
other  fundamental  discoveries  have  sometimes  met. 
In  the  present  case  there  was  no  opposition  to  the 
principles  involved  in  Mendel's  discovery,  for  Darwin's 
great  work  on  " Animals  and  Plants"  (1868),  that  dealt 
largely  with  problems  of  heredity,  was  widely  read  and 
appreciated.  True,  Mendel's  paper  was  printed  in 
the  journal  of  a  little  known  society  —  the  Natural 
History  Society  of  Briinn,  —  but  we  have  documentary 
evidence  that  his  results  were  known  to  one  at  least  of 
the  leading  botanists  of  the  time. 

It  was  during  these  years  that  the  great  battle  for 
evolution  was  being  fought.  Darwin's  famous  book  on 
"The  Origin  of  Species"  (1859)  overshadowed  all  else. 
Two  systems  were  in  deadly  conflict  —  the  long-ac- 
cepted doctrine  of  special  creation  had  been  challenged. 
To  substitute  for  that  doctrine  the  theory  of  evolution 
seemed  to  many  men  of  science,  and  to  the  world  at 
large,  like  a  revolution  in  human  thought.  It  was  in 
fact  a  great  revolution.  The  problems  that  bore  on  the 

73 


74  HEREDITY  AND   SEX 

question  of  how  the  higher  animals  and  plants,  and 
man  himself,  arose  from  the  lower  forms  seemed  the 
chief  goal  of  biological  work  and  thought.  The  out- 
come was  to  establish  the  theory  of  evolution.  The 
circumstantial  evidence  that  was  gathered  seemed  so 
fully  in  accord  with  the  theory  of  evolution  that  the 
theory  became  widely  accepted.  The  acute  stage  was 
passed,  and  biologists  found  themselves  in  a  position 
to  examine  with  less  haste  and  heat  many  other  phe- 
nomena of  the  living  world  equally  as  important  as 
evolution. 

It  gradually  became  clear,  when  the  clouds  of  con- 
troversy had  passed,  that  what  I  have  ventured  to  call 
the  "  circumstantial  evidence  "on  which  the  theory  of 
evolution  so  largely  rested,  would  not  suffice  as  a  direct 
proof  of  evolution.  Investigation  began  to  turn  once 
more  to  that  field  of  observation  where  Darwin  had 
found  his  inspiration.  The  causes  of  variations  and 
the  modes  of  inheritance  of  these  variations,  the  very 
foundations  of  the  theory  of  evolution,  were  again 
studied  in  the  same  spirit  in  which  Darwin  himself  had 
studied  them.  The  return  to  Darwin's  method  rather 
than  to  Darwin's  opinions  marks  the  beginning  of  the 
new  era. 

In  1900  three  botanists  were  studying  the  problem 
of  heredity.  Each  obtained  evidence  of  the  sort 
Mendel  had  found.  Happily,  Mendel's  paper  was 
remembered.  The  significance  of  his  discovery  now 
became  apparent.  De  Vries,  Correns,  and  Tschermak 
brought  forward  their  evidence  in  the  same  year  (1900). 
Which  of  the  three  first  found  Mendel  cannot  be  stated, 
and  is  of  less  importance  than  the  fact  that  they  ap- 


THE   MENDELIAN   PRINCIPLES  OF  HEREDITY      75 

predated  the  significance  of  his  work,  and  realized 
that  he  had  found  the  key  to  the  discoveries  that  they 
too  had  made.  From  this  time  on  the  recognition  of 
Mendel's  discovery  as  of  fundamental  importance  was 
assured.  Bateson's  translation  of  his  paper  made 
Mendel's  work  accessible  to  English  biologists,  and 
Bateson's  own  studies  showed  that  Mendel's  principles 
apply  to  animals  as  well  as  to  plants. 

THE    HEREDITY   OF    ONE    PAIR    OF    CHARACTERS 

Mendel's  discovery  is  sometimes  spoken  of  as  Men- 
del's Principles  of  Heredity  and  sometimes  as  Mendel's 
Law.  The  former  phrase  gives  a  better  idea  perhaps 
of  what  Mendel  really  accomplished,  for  it  is  not  a  little 
difficult  to  put  his  conclusions  in  the  form  of  a  law. 
Stated  concisely  his  main  discovery  is  this :  —  in  the 
germ-cells  of  hybrids  there  is  a  free  separation  of  the 
elements  derived  from  the  two  parents  without  regard  to 
which  parent  supplied  them.^ 

An  example  will  make  this  more  obvious.  Mendel 
crossed  an  edible  pea  belonging  to  a  race  with  yellow 
seeds  to  a  pea  belonging  to  a  race  with  green  seeds 
(Fig.  42).  The  offspring  or  first  filial  generation  (Fi) 
had  seeds  all  of  which  were  yellgw.  When  the  plants 
that  bore  these  seeds  were  self-fertilized,  there  were 
obtained  in  the  next  generation,  F2r  both  yellow  and 
green  peas  in  the  proportion  of  3  yellows  to  1  green 
(Fig.  42).  This  is  the  well-known  Mendelian  ratio 
of  3:1. 

The  clue  to  the  meaning  of  this  ratio  was  found  when 
the  plants  of  the  second  generation  (F2)  were  selfbred. 
The  green  peas  bred  true ;  but  the  yellows  were  of  two 


76 


HEREDITY  AND   SEX 


kinds  —  some  produced  yellow  and  green  seeds  again 
in  the  ratio  of  3  :  1 ;  others  produced  only  yellow 
peas.  Now,  if  the  yellows  that  bred  true  were  counted, 
it  was  found  that  the  number  was  but  one-third  of 
all  the  yellows. 


FIG.  42.  —  Illustrating  Mendel's  cross  of  yellow  (lighter  color)  and  greeu 
(dark  color)  peas. 


THE   MEXDELIAX  PRINCIPLES  OF  HEREDITY      77 

In  other  words,  it  was  shown  that  the  ratio  of  3  yel- 
lows to  1  green  was  made  up  of  1  pure  yellow,  2  hy- 
brid yellows,  1  pure  green.  This  gave  a  clue  to  the 
principles  that  lay  behind  the  observed  results. 

Mendel's  chief  claim  to  fame  is  found  in  the  discovery 
of  a  simple  principle  by  means  of  which  the  entire 
series  of  events  could  be  explained.  He  pointed  out 
that  if  the  original  parent  with  yellow  (Pi)  carried 
something  in  the  germ  that  made  the  seed  yellow,  and 
the  original  parent  with  green  seeds  (Pi)  carried  some- 
thing that  made  the  seed  green,  the  hybrid  should  con- 
tain both  things.  If  jboth  being  present^  one  domi- 
nates the  other  or  gives  color  to  the  pea,  all  the  peas  in 
the  hybrid  generation  will  be  of  one  color  —  yellow  in 
this  case.  Mendel  assumed  that  in  the  germ-cells  of 
these  hybrids  the  two  factors  that  make  yellow  and 
green  separate,  so  that  half  of  the  germ-cells  contain 
yellow-producing  material,  and  half  contain  green- 
producing  material.  This  is  Mendel's  principle  of 
separation  or  segregation.  It  is  supposed  to  occur 
both  in  the  male  germ-cells  of  the  hybrid  flower,  i.e. 
in  the  anthers,  and  also  in  the  ovules.  If  self-fertili- 
zation occurs  in  such  a  plant,  the  following  combina- 
tions are  possible :  A  yellow-bearing  pollen  grain  may 
fertilize  a  " yellow"  ovule  or  it  may  fertilize  a  "green" 
ovule.  The  chances  are  equal.  If  the  former  occurs, 
a  pure  yellow-seeded  plant  will  result ;  if  the  latter  a 
hybrid  yellow-seeded  plant.  The  possible  combina- 
tions for  the  green-producing  pollen  are  as  follows :  A 
" green"  pollen  grain  may  fertilize  a  "yellow"  ovule, 
and  produce  a  hybrid,  yellow-seeded  plant,  or  it  may 
fertilize  a  "green"  ovule,  and  produce  a  green-seeded 


78  HEREDITY  AND   SEX 

plant.  If  these  meetings  are  random,  the  general  or 
average  outcome  will  be :  1  pure  yellow,  2  hybrid 
yellows,  and  1  pure  green. 

It  is  now  apparent  why  the  pure  yellows  will  always 
breed  true,  why  the  yellow-greens  will  split  again  into 
yellows  and  greens  (or  1:2:1),  and  why  the  pure 
greens  breed  true.  By  this  extremely  simple  assump- 
tion the  entire  outcome  finds  a  rational  explanation. 


o 


F.1 


t 


0 


FIG.  43.  —  "  Checker  "  diagram  to  show  segregation  and  recombination  of 
factors.  In  upper  line,  a  black  bearing  gamete  is  supposed  to  unite  with  a 
white  bearing  gamete  to  give  the,  zygotes  shown  in  FI,  each  of  which  is 
heterozygous  for  black-white  here  represented  as  allelomorphs,  etc. 

The  same  scheme  may  be  represented  by  means  of 
the  above  "  checker"  diagram  (Fig.  43).  Black  crossed 
to  white  gives  hybrid  black,  F\,  whose  germ-cells  are 
black  or  white  after  segregation.  The  possible  com- 
bination of  these  on  random  meeting  at  the  time  of 
fertilization  is  shown  by  the  arrows  in  FI  and  the  results 
are  shown  in  the  line  marked  F2.  There  will  be  one 
pure  black,  to  two  black-and-whites,  to  one  pure  white. 


THE  MENDELIAX   PRINCIPLES  OF  HEREDITY      79 

The  first  and  the  last  will  breed  true,  if  self-fertilized, 
because  they  are  pure  races,  while  the  black-and-whites 
will  give  once  again,  if  inbred,  the  proportions  1:2:  1. 
A  better  illustration  of  Mendel's  principles  is  shown 
in  the  case  of  the  white  and  red  Mirabilis  jalapa  de- 
scribed by  Correns.  This  case  is  illustrated  in  Fig.  44, 


FIG.  44.  —  Cross  between  white  and  red  races  of  Mirabilis  Jalapa,  giving 
a  pink  hybrid  in  FI  which  when  inbred  gives,  in  F2,  1  white,  2  pink,  1  red. 

in  which  the  red  flower  is  represented  in  black  and  the 
pink  is  in  gray.  The  hybrid,  F^  out  of  white  by  red, 
has  pink  flowers,  i.e.  it  is  intermediate  in  color.  When 
these  pink  flowers  are  self-fertilized  they  produce 
1  white,  2  pink,  and  1  red-flowered  plant  again.  The 
history  of  the  germ-cells  is  shown  in  Fig.  45.  The  germ- 


st 


80  HEREDITY  AND   SEX 

cell  of  the  F\  pink  flower  segregates  into  " white"  and 
"red,"  which  by  chance  combination  give  the  white- 
pink,  and  red  flowers  of  F2.  The  white  and  red  flowers 
are  pure ;  the  pink  heterozygous,  i.e.  hybrid  or  mixed. 
In  this  case  neither  red  nor  white  dominates,  so  that  the 
hybrid  can  be  distinguished  from  both  its  parents. 


PARENTS 


F'O-  *-^*  • 

-6    o    o    o    •    • 

FIG.  45.  —  Illustrating  history  of  gametes  in  cross  shown  in  Fig.  44.  A 
white  and  a  red  bearing  gamete  unite  to  form  the  pink  zygote  in  F\,  whose 
gametes,  by  segregation,  are  red  and  white,  which  by  random  combinations 
give  the  F2  zygotes,  etc. 

Mendel  tested  his  hypotheses  in  numerous  ways,  that 
I  need  not  now  discuss,  and  found  in  every  case  that 
the  results  coincided  with  expectation. 

THE    HEREDITY   OF   A   SEX-LINKED    CHARACTER 

We  are  now  in  a  position  to  see  how  Mendel's  funda- 


THE  MENDELIAN  PRINCIPLES  OF  HEREDITY      81 

mental  principle  of  segregation  applies  to  a  certain  class 
of  characters  that  in  the  last  chapter  I  called  "  sex- 
linked"  characters. 

Diagram  35  (page  64)  will  recall  the  mode  of  trans- 
mission of  one  of  these  characters,  viz.  white  eyes. 

Let  us  suppose  that  the  determiner  for  white  eyes 
is  carried  by  the  sex  chromosome.  *The  white-.eyed 
male  has  one  sex  chromosome  of  this  kind.  This  sex 
chromosome  passes  into  the  female-producing  spermato- 
zoon. 

Such  a  spermatozoon  fertilizing  an  egg  of  the  red- 
eyed  fly  gives  a  female  with  two  sex  chromosomes  — 
one  capable  of  producing  red,  one  capable  of  producing 
white.     The  presence  of  one  red-producing  chromosome 
suffices  to  produce  a  red-eyed  individual. 

When  the  FI  female  produces  her  eggs,  the  two  sex 
chromosomes  separate  at  one  of  the  two  maturation 
divisions.  Half  of  the  eggs  on  an  average  will  contain 
the  "white"  sex  chromosome,  half  the  "red."  There 
are,  then,  two  classes  of  eggs. 

When  the  FI  male  produces  his  sperm,  there  are 
also  two  classes  of  sperm  —  one  with  the  "red" 
sex  chromosome  (the  female-producing  sperm),  and 
one  without  a  sex  chromosome  (the  male-producing 
sperm) . 

Chance  meeting  between  eggs  and  sperm  will  give 
the  classes  of  individuals  that  appear  in  the  second  filial 
generation  (F2) .  It  will  be  observed  that  the  Mendelian 
ratio  of  3  red  to  1  white  appears  here  also.  Segregation 
gives  this  result. 

The  explanation  that  has  just  been  given  rests  on 
the  assumption  that  the  mechanism  that  brings  about 


82  HEREDITY  AND  SEX 

the  distribution  of  the  red-  and  the  white-producing 
factors  is  the  same  mechanism  that  is  involved  in  sex 
determination.  On  this  assumption  we  can  readily 
understand  that  any  character  that  is  dependent  on  the 
sex  chromosomes  for  its  realization  will  show  sex-linked 
inheritance. 

The  reciprocal  cross  (Fig.  36)  is  equally  instructive. 
If  a  white-eyed  female  is  mated  to  a  red-eyed  male, 
all  the  daughters  are  red-eyed  like  the  father,  and  all 
the  sons  are  white-eyed  like  the  mother.  When  these, 
FI,  flies  are  bred  to  each  other  there  are  produced  red- 
eyed  females  (25%),  white-eyed  females  (25%),  red- 
eyed  males  (25%),  and  white-eyed  males  (25%).  The 
explanation  (Fig.  36 ;  page  65)  is  in  principle  the 
same  as  for  the  other  cross.  If,  for  instance,  we 
assume  that  the  two  X  chromosomes  in  the  white-eyed 
female  carry  the  factors  for  white,  all  the  eggs  will 
carry  one  white-producing  X.  The  red-eyed  male  will 
contain  one  X  chromosome  which  is  red-producing 
and  passes  into  the  female-producing  sperm.  The 
other  sperm  will  not  contain  any  sex  chromosome,  and 
hence  lacks  the  factors  for  these  eye  colors.  When  the 
female-producing  sperm,  that  carries  the  factor  for 
red,  fertilizes  a  " white"  egg,  the  egg  will  give  rise  to  a 
female  with  red  eyes,  because  of  the  presence  of  one 
red-producing  chromosome.  When  the  male-produc- 
ing sperm  fertilizes  any  egg,  a  white-eyed  son  will  be 
produced,  because  the  single  sex  chromosome  which 
he  gets  from  his  mother  is  white-producing. 

The  production  of  four  classes  of  individuals  in  the 
second  generation  works  out  on  the  same  scheme,  as 
shown  in  the  diagram.  The  inheritance  of  white  and 


THE  MEXDELIAX   PRINCIPLES  OF  HEREDITY      83 

red  eyes  in  these  cases  is  typical  of  all  sex-linked  in- 
heritance. In  man,  for  instance,  color  blindness,  so 
common  in  males  and  rare  in  females,  follows  the 
same  rules.  It  appears  that  hemophilia  in  man  and 
night-blindness  are  also  examples  of  sex-linked  in- 
heritance. These  cases,  as  already  stated,  were  formerly 
included  under  the  term  "sex-limited  inheritance,"  that 
implies  that  a  character  is  limited  to  one  sex,  but  we 
now  know  that  characters  such  as  these  may  be  trans- 
ferred to  the  females,  hence  the  term  is  misleading. 
Their  chief  peculiarity  is  that  in  transmission  they  ap- 
pear as  though  linked  to  the  factor  for  sex  contained  in 
the  sex  chromosome,  hence  I  prefer  to  speak  of  them  as 
sex-linked  characters. 

If  our  explanation  is  well  founded,  each  sex-linked 
character  is  represented  by  some  substance  —  some 
material  particle  that  we  call  a  factor  in  the  sex 
chromosome.  There  may  be  hundreds  of  such  materials 
present  that  are  essential  for  the  development  of  sex- 
linked  characters  in  the  organism. 

The  sex  chromosomes  must  contain,  therefore,  a 
large  amount  of  material  that  has  nothing  whatever 
to  do  with  sex  determination;  for  the  characters  in 
question  are  not  limited  to  any  particular  sex,  although 
in  certain  combinations  they  may  appear  in  one  sex 
and  not  in  the  other. 

What  then,  have  the  sex  chromosomes  to  do  with  sex  ? 
The  answer  is  that  sex,  like  any  other  character,  is  due 
to  some  factor  or  determiner  contained  in  these  chro- 
mosomes. It  is  a  differential  factor  of  such  a  kind 
that  when  present  in  duplex,  as  when  both  sex  chromo- 
somes are  present,  it  turns  the  scale  so  that  a  female 


84  HEREDITY  AND   SEX 

is  produced  —  when  present  in  simplex,  the  result  is 
to  produce  a  male. 

In  other  words,  it  is  not  the  sex  chromosomes  as  a 
whole  that  determine  sex,  but  only  a  part  of  these  chro- 
mosomes. Hence  we  can  understand  how  sex  is  deter- 
mined when  an  unequal  pair  of  chromosomes  is  pres- 
ent, as  in  lygaeus.  The  smaller  chromosome  lacks 
the  sex  differential,  and  probably  a  certain  number  of 
other  -materials,  so  that  sex-linked  inheritance  is  pos- 
sible here  also.  Moreover,  in  a  type  like  oncopeltus, 
where  the  two  sex  chromosomes  are  alike  in  size,  we 
infer  that  they  too  differ  by  the  sex  differential.  If 
all  the  other  factors  are  present,  as  their  size  suggests, 
sex-linked  inheritance  of  the  same  kind  would  not  be 
expected,  but  the  size  difference  observable  by  the 
microscope  is  obviously  too  gross  to  make  any  such 
inference  certain.  We  have  come  to  see  that  it  was  a 
fortunate  coincidence  only  that  made  possible  the  dis- 
covery of  sex  determination  through  the  sex  chromo- 
somes, because  the  absence  of  the  sex  factor  alone  in 
the  Y  chromosome  might  have  left  that  chromosome 
in  the  male  so  nearly  the  same  size  as  the  X  in  the 
female  that  their  relation  to  sex  might  never  have  been 
suspected.  When,  however,  one  of  the  sex  chromosomes 
began  to  lose  other  materials,  it  became  possible  to 
identify  it  and  discover  that  sex  is  dependent  upon 
its  distribution. 

THE    HEREDITY    OF   TWO    PAIRS    OF    CHARACTERS 

Mendel  observed  that  his  principles  of  heredity  apply 
not  only  to  pairs  of  characters  taken  singly,  but  to 
cases  where  two  or  more  pairs  of  characters  are  involved. 


THE   MEXDELIAX   PRINCIPLES  OF  HEREDITY      85 

An  illustration  will  make  this  clear.  There  are  races 
of  edible  peas  in  which  the  surface  is  round ;  other  races 
in  which  the  surface  is  wrinkled.  Mendel  crossed  a 
pea  that  produces  yellow  round  seeds  with  one  that  pro- 
duces wrinkled  green  seeds. 

The  result  of  this  cross  was  a  plant  that  produced 
yellow  round  peas  (Fig.  46).  Both  yellow  and. round 
are  therefore  dominant  characters.  When  these  F\ 
plants  were  self-fertilized,  there  were  produced  plants 
some  of  which  bore  yellow  round  peas,  some  yellow 
wrinkled  peas,  some  green  round  peas  and  some  green 
wrinkled  peas.  These  were  in  the  proportion  of 
9:3:3:1. 

The  explanation  of  the  result  is  as  follows :  One  of 
the  original  plants  produced  germ-cells  all  of  which 
bore  determiners  for  yellow  and  for  round  peas,  YR;  the 
other  parent  produced  gametes  all  of  which  bore  deter- 
miners for  green  and  for  wrinkled,  GW  (Fig.  47). 
Their  combination  may  be  represented  : 

YR    by    GW  =  YRGW 

The  germ-cells  of  the  hybrid  plant  YRGW  produced 
germ-cells  (eggs  and  pollen)  that  have  either  Y  or  G, 
and  R  or  W.  Expressed  graphically  the  pairs,  the 
so-called  allelomorphs,  are : 

Y  R^ 

G  W 

and  the  only  possible  combinations  are  YR,  YW,  GR, 
GW.  When  pollen  grains  of  these  four  kinds  fall  on 
the  stigma  of  the  same  kind  of  hybrid  plant  whose 
ovules  are  also  of  the  four  kinds  the  following  chance 
combinations  are  possible : 


HEREDITY  AND   SEX 


YR 
YR 

YR 
YW 

YR 
GR 

YR 
GW 

YW 
YR 

YW 
YW 

YW 
GR 

YW 
GW 

GR 
YR 

GR 
YW 

GR 
GR 

GR 
GW 

GW 
YR 

GW 
YW 

GW 
GR 

GW 
GW 

FIG.  46.  —  Illustrating  Mendel's  cross  of  yellow-round  with  green-wrinkled 
peas.  The  figures  show  the  peas  of  F\  and  F2  in  the  latter  in  the  charac- 
teristic ratio  of  9  :  3  :  3  :  1. 


THE  MEXDELIAX    PRINCIPLES   OF  HEREDITY      87 


o 


PARENTS 

Y  R    X  /  G~W 


O 

YR  GW 


'YR 


YW 
YW 


GR 
GR 


GW 

GW. 


FIG.  47.  —  Illustrating  the  history  of  the  gametes  of  the  cross  represented 
in  Fig.  46.  The  composition  of  the  parents  YR  and  GW  and  of  the  FI  hybrid 
YRGW  is  given  above.  The  four  classes  of  ovules  and  of  pollen  are  given  in 
the  middle  of  the  figure.  These  by  random  combinations  give  the  kinds  of 
zygotes  represented  iri  the  squares  below. 


88  HEREDITY  AND   SEX 

In  each  combination  in  the  table  the  character  of 
the  plant  is  determined  by  the  dominant  factors,  in 
this  case  yellow  and  round,  hence  : 

9  YR  :  3  YW  :  3  GR  :  1  GW 

This  result  works  out  on  the  assumption  that  there 
is  independent  assortment  of  the  original  determiners 
that  entered  into  the  combination.  The  determiner 
for  yellow  and  the  determiner  for  round  peas  are 
assumed  to  act  independently  and  to  segregate  from 
green  and  wrinkled  that  entered  from  the  other  parent. 
The  9:3:3:1  ratio  rests  on  this  assumption  and  is  the 
actual  ratio  realized  whenever  two  pairs  of  characters 
freely  Mendelize. 

THE    HEREDITY   OF   TWO    SEX-LINKED    CHARACTERS 

The  inheritance  of  two  sex-linked  characters  may  be 
illustrated  by  an  imaginary  case  in  which  the  linkage 
of  the  factors  to  each  other  is  ignored.  Then  the  same 
case  may  be  given  in  which  the  actual  results  obtained, 
involving  linkage,  are  discussed. 

The  factors  in  the  fruit  fly  for  gray  color,  G,  and  for 
red  eye,  R,  are  both  sex-linked,  i.e.  contained  in  the 
X  chromosome.  Their  allelomorphs,  viz.,  yellow  color, 
Y,  and  white  eye,  W,  are  also  sex-linked.  When  a 
gray  red-eyed  female  is  mated  to  a  yellow  white-eyed 
male,  the  daughters  and  sons  are  gray-red,  GR.  Their 
origin  is  indicated  in  the  following  scheme  : 

Gray-red  $  G  R   X  —  GR    X 

Yellow-white  <?  Y  W  X-        ... 

G  R   X  Y   W  X   Gray-red  $ 
G  R   X    .     .     .       Gray-red* 


THE  MENDELIAN   PRINCIPLES  OF  HEREDITY      89 

In  the  gray-red  F\  female  there  will  be  the  possibility 
of  interchange  of  the  Cf  and  Y,  and  of  the  W  and  R 
factors,  so  that  gametes  of  four  kinds  will  be  formed, 
namely,  GRX  -  GWX  YRX  -  YWX.  For  the 
moment  we  may  assume  free  interchange  •  of  factors ; 
and  therefore  these  four  classes  of  eggs  will  exist  in 
equal  numbers. 

In  the  gray-red  FI  male  there  is  but  one  X  chromo- 
some that  contains  the  factors  G  and  R.  There  will 
be  then  only  one  kind  of  female-producing  sperm, 
viz.,  GRX',  and  one  kind  of  male-producing  sperm, 
the  latter  containing  no  X,  and  therefore  none  of  the 
factors  in  question.  The  chance  meeting  of  these  two 
classes  of  sperm  and  the  four  classes  of  eggs  gives  the 
following  results : 

Fi    eggs      GRX  —  GWX  —  YRX  —  YWX 
FI  sperm       GRX 

Females.  Males. 

GRXGRX     gray-red.-  GRX    gray-red. 

GRXGWX     gray-red.-  GWX    gray-white. 

GRXYRX      gray-red.-  YRX    yellow-red. 

GRXYWX     gray-red.-  YWX   yellow-white. 

All  the  females  are  gray  with  red  eyes,  since  these 
are  the  dominant  characters.  There  are  four  classes 
of  males.  These  males  give  a  measure  of  the  kinds  of 
eggs  produced  by  the  females,  since  the  male-producing 
sperms,  having  no  sex  chromosomes,  do  not  affect 
the  sex-linked  characters  derived  through  the  sex 
chromosome  of  the  FI  female.  The  expected  proportion 
is  therefore : 


90 


HEREDITY  AND   SEX 


GR? 
4 


GWZ 
1  1 


1 


These  results  are  illustrated  by  means  of  Fig.  48, 
in  which  the  yellow  color  of  the  fly  is  indicated  by 
stippling  the  body  and  wings,  and  the  red  eyes  by 
black.  The  X  chromosome  is  also  marked  and  colored 


Parents 


xx 


XX  X 


FIG.  48.  —  Inheritance  of  yellow-white  ( $ )  and  gray-red  ( 9 )  of  Dro- 
sophila.  In  F\  both  sexes  are  gray-red.  In  F2  are  produced  4  GR  9  — 
1  GR  $  —  1  GW  $  —  I  YR  $  —  1  YW  $. 


THE  MENDELIAN   PRINCIPLES  OF  HEREDITY      91 

in  the  same  way  as  the  flies ;  thus  the  two  X's  in  the 
red-eyed  gray  female  are  half  black  (for  red)  and  half 
gray;  the  single  X  in  the  white-eyed  yellow  male  is 
half  white  and  half  stippled. 

In  the  FI  generation  the  X  chromosomes  are  first 
represented  as  they  came  in  (second  line),  i.e.  with 
their  original  composition.  The  next  line  gives  the 
three  large  classes  that  result,  viz.,  2  GR9  --1  GR$ 
- 1  YW  $  .  But  if  free  interchange  takes  place  in  the 
female,  some  of  the  eggs  will  have  chromosomes  like 
those  in  the  fourth  line,  viz.  YR  and  GW.  Such 
eggs  will  give  the  classes  represented  in  the  lowest  line, 
viz.,  2  GR9—  1  GW$—  1  YR$.  Thus,  as  already 
explained,  there  results  one  kind  of  female  and  four 
kinds  of  males. 

I  said  that  the  proportion  4:1:1:1:1  is  the  ideal 
result  in  the  cross  between  the  yellow-white  and  the 
gray-red  flies.  This  ideal  scheme  is  not  realized  because 
of  a  complication  that  comes  in.  The  complication 
is  due  to  linkage  or  a  tendency  to  hang  together  of  the 
characters  that  go  in  together.  We  must  now  take  up 
this  question.  It  is  one  of  the  most  modern  develop- 
ments of  the  Mendelian  theory  —  one  that  at  first 
seemed  to  throw  doubt  on  the  fundamental  idea  of 
random  assortment  that  gives  Mendel's  proportion 
9:3:3:1.  But  I  believe  we  can  now  offer  a  reasonable 
explanation,  which  shows  that  we  have  to  do  here  with 
an  extension  of  Mendelism  that  in  no  sense  invalidates 
Mendel's  principle  of  segregation.  It  not  only  extends 
that  principle,  as  I  have  said,  but  gives  us  an  oppor- 
tunity to  analyze  the  constitution  of  the  germ-plasm 
in  a  way  scarcely  dreamed  of  two  or  three  years  ago. 


92  HEREDITY  AND   SEX 

The  actual  numbers  obtained  in  the  GR  by  YW 
cross  are  as  follows.  These  are  the  figures  that  Dexter 
has  obtained : 

GR?         GR$  GW$  YR$          YW  $ 

6080          2870  36  34  2373 

The  apparent  discrepancy  between  the  expected 
and  the  realized  ratios  is  due  to  the  linkage  of  the  factors 
that  went  into  the  cross.  For  instance,  the  factors 
for  gray  and  red  that  went  in  with  one  chromosome  are 
linked ;  likewise  their  allelomorphs,  yellow  and  white. 
As  shown  by  the  analysis,  the  F\  female  offspring 
will  have  two  sex  chromosomes,  one  of  each  sort  - 
one  from  the  father,  the  other  from  the  mother.  But 
the  male  will  have  but  one  sex  chromosome  derived 
from  the  mother. 

If  in  the  germ-cells  of  the  F\  females  there  were 
random  assortment  of  the  allelomorphs  in  the  sex 
chromosomes,  the  ideal  ratio  of  4:1:1:1:1  would,  as 
has  been  said,  be  realized.  But  if  the  red  and  gray 
factors  tend  to  remain  together  since  they  go  in 
together  in  the  one  chromosome,  and  the  yellow  and 
white  in  the  other  chromosome  tend  to  remain  together, 
and  if  the  chances  are  about  84  to  1  that  the  factors 
that  enter  together  remain  together,  the  realized  ratio 
of  170  :  84  :  1 :  1 :  84  will  be  found. 

Experiments  show  that,  for  these  two  factors,  the 
chances  are  about  84  to  1  that  the  factors  that  go  in 
together  remain  together;  hence  the  departure  from 
Mendel's  ratios  for  these  two  pairs  of  characters.  We 
may  make  a  general  statement  or  hypothesis  that 
covers  cases  like  these,  and  in  fact  all  cases  where 


THE  MENDELIAN  PRINCIPLES   OF  HEREDITY      93 


linkage  occurs :  viz.  that  when  factors  lie  in  different 
chromosomes  they  freely  assort  and  give  the  Mendelian 
expectation  ;  but  when  factors  lie  in  the  same  chromo- 
some, they  may  be  said  to  be  linked  and  they  give 
departures  from  the  Mendelian  ratios.  The -extent  to 
which  they  depart  from  expectation  will  vary  with 
different  factors.  I  have  suggested  that  the  departures 
may  be  interpreted  as  the  distance  between  the  factors 
in  question. 

A    THEORY   OF    LINKAGE 

In  order  to  understand  more  fully  what  is  meant 
by  linkage  on  the  interpretation  that  I  have  here 
followed,  it  will  be  necessary  to  consider  certain  changes 
that  take  place  in  synapsis.  The  sex  chromosomes 


o* 


FIG.  49.  —  Illustrating  chiasma-type  theory.  1  and  2,  from  Triton 
cristatus,  3—46,  chromosomes  of  Batracoseps  attenuatus.  Note  especially 
the  chiasma  shown  in  13.  (After  Janssens). 


94 


HEREDITY  AND   SEX 


(when  two  are  present  as  in  the  female),  like  all  other 
chromosomes,  unite  in  pairs  at  the  synaptic  period. 
A  recognized  method  of  uniting  is  for  like  chromosomes 
to  come  to  lie  side  by  side. 

Before  they  separate,  as  they  do  at  one  of  the  two 
maturation  divisions,  each  chromosome  may  be  seen 
to  be  split  throughout  the  length.  Thus  there  are 


FIG.  50.  —  Chromatin  filaments  in  the  amphitene  stage  from  spermato- 
cytes  of  Batracoseps.     (After  Janssens.) 

formed  four  parallel  strands  each  equivalent  to  a 
chromosome  —  the  tetrad  group.  At  this  time  Jans- 
sens  has  found  that  cross  unions  between  the  strands 
are  sometimes  present  (Fig.  49).  In  consequence 
a  strand  is  made  up  of  a  part  of  one  chromosome  and 
a  part  of  another.  Whether  this  cross  union  can  be 
referred  to  an  earlier  stage  —  at  the  time  when  the  two 
like  chromosomes  come  together,  when  they  can  be 


THE   MEXDELIAX    PRINCIPLES   OF  HEREDITY      95 

seen  to  twist  around  each  other  (Fig.  50)  —  is  not 
certain ;  but  the  fact  of  the  existence  of  cross  connec- 
tions is  the  important  point.  A  consequence  of  this 
condition  is  that  the  chromosomes  that  come  out  of 
the  tetrad  may  represent  different  combinations  of 
those  that  united  to  form  the  group.  On  the  basis 
of  this  observation  we  can  explain  the  results  of  associ- 
ated inheritance.  For,  to  the  same  extent  to  which 
the  chromosomes  that  unite  remain  intact,  the  factors 
are  linked,  and  to  the  extent  to  which  crossings  occur 
the  exchange  of  factors  takes  place.  On  the  basis 
of  the  assumption  of  the  linear  arrangement  of  the 
factors  in  the  chromosomes  the  distance  apart  of  the 
factors  is  a  matter  of  importance.  If  two  factors 
lie  near  together,  the  chance  of  a  break  occurring  be- 
tween them  is  small  in  proportion  to  their  nearness. 
We  have  found  that  some  factors  cross  over  not  once 
in  a  hundred  times.  I  interpret  this  to  mean  that  they 
lie  very  near  together  in  the  chromosome. 

Other  factors  cross  over  to  various  degrees;  in  the 
extreme  cases  the  chance  is  one  to  one  that  they  cross 
over.  In  such  cases  the  factors  lie  far  apart  —  perhaps 
near  the  ends  of  the  chromosome. 

The  strongest  evidence  in  favor  of  this  view  is  found 
when  the  constant  relation  of  the  factors  to  each  other 
is  considered.  If,  for  instance,  we  know  the  distance 
from  A  to  B  (calculated  on  the  basis  of  crossing  over) 
and  from  B  to  C,  we  can  predict  what  A  and  C  will  do 
when  they  are  brought  into  the  hybrid  from  two 
parents.  If  a  fourth  factor,  Z),  is  discovered  and  its 
distance  from  A  is  made  out,  we  can  predict  before  the 
experiment  is  made  what  will  take  place  when  D  and 


96  HEREDITY  AND   SEX 

B  or  D  and  C  are  combined.  The  prediction  has  been 
fulfilled  so  many  times  and  in  so  many  ways  that  we 
feel  some  assurance  that  we  have  discovered  here  a 
working  hypothesis  of  considerable  interest.  If  the 
hypothesis  becomes  established,  it  will  enable  us  to 
analyze  the  structure  of  the  chromosomes  themselves 
in  the  sense  that  we  can  determine  the  relative  location 
of  factors  in  the  chromosomes.  If,  as  seems  not 
improbable,  the  chromosomes  are  the  most  important 
element  in  Mendelian  inheritance,  the  determination 
of  the  linear  series  of  factors  contained  in  them  becomes 
a  matter  of  great  theoretical  interest ;  for  we  gain 
further  insight  into  the  composition  of  the  material 
on  which  heredity  itself  depends. 

There  is  a  corollary  to  this  explanation  of  crossing 
over  that  has  a  very  direct  bearing  on  the  results.  In 
the  male  there  is  only  one  X  chromosome  present. 
Hence  crossing  over  is  impossible.  The  experimental 
results  show  that  no  crossing  over  takes  place  for 
sex-linked  factors  in  the  male  of  drosophila. 

Other  factors,  however,  lie  in  other  chromosomes. 
In  these  cases  the  chromosomes  exist  in  pairs  in  the 
male  as  well  as  in  the  female.  Does  crossing  over 
occur  here  in  both  sexes?  Let  me  illustrate  this  by 
an  example.  In  drosophila  the  factor  for  black 
body  color  and  the  factor  that  gives  vestigial  wings 
lie  in  the  same  chromosome,  which  we  may  call  the 
second  chromosome.  If  a  black,  long-winged  female 
is  crossed  with  a  gray  vestigial  male,  all  the  offspring 
will  be  gray  in  color  and  have  long  wings,  since  these 
are  the  dominant  characters.  If  these  F\  flies  are 
inbred,  the  following  classes  will  appear: 


THE  MENDELIAN   PRINCIPLES  OF  HEREDITY      97 

Gray  Long          Black  Long          Gray  Vestigial 
2316  1146  737 

It  will  be  noted  that  there  are  no  black  vestigial 
flies.  Their  absence  can  be  explained  on  the  assump- 
tion that  no  crossing  over  in  the  male,  between  the 
factors  in  the  second  chromosome,  has  taken  place. 

But  if  another  generation  (F3)  is  raised,  some  black 
vestigial  flies  appear.  With  these,  it  is  possible  to  test 
the  hypothesis  just  stated.  If,  for  instance,  some  of 
the  long,  gray,  FI  females  are  mated  to  black  vestigial 
males,  the  following  classes  are  produced : 

GL  BL  GV  BV 

578  1413  1117  307 

The  results  are  explicable  on  the  view  that  crossing 
over  takes  place  in  the  germ-cell  of  the  FI  female, 
and  that  the  chance  that  such  will  occur  is  as  1  to  3. 

But  if  the  long-winged,  gray,  FI  males  are  crossed 
to  black  vestigial  females,  only  the  following  classes 
are  produced : 

BL  GV 

992  721 

These  results  are  in  accord  with  the  hypothesis  that 
no  crossing  over  takes  place  between  the  second 
chromosomes  in  the  FI  male.  Why  crossing  over 
should  occur  in  the  FI  female,  and  not  in  the  FI 
male,  we  do  not  know  at  present ;  and  until  the 
synaptic  stages  in  the  males  and  females  have  been 
carefully  studied,  we  must  wait  for  an  answer  to  the 
question. 


98 


HEREDITY  AND   SEX 


THREE    SEX-LINKED    FACTORS 

When  three  sex-linked  factors  exist  in  the  same 
chromosomes,  we  have  a  method  by  means  of  which 
the  "  crossing-over  "  hypothesis  may  be  put  to  a  further 
test.  Sturtevant  has  recently  worked  over  the  evidence 


I  £7 


G.' 


H.' 


FIG.  51.  —  A-D,  YW  and  GR  that  enter  (A),  crossing  over  to  give  YR 
and  GW  as  seen  in  D.  E-Ei,  no  crossing  over.  F-Fi,  crossing  between  WM 
and  RL.  G-Gi,  crossing  between  YW  and  GR.  H-Hi,  double  crossing  over 
of  YWM  and  GRL,  to  give  YRM  and  GWL. 

for  a  case  of  this  kind.  He  analyzed  the  data  of  the 
cross  between  a  fly  having  gray  color,  red  eyes,  long 
wings,  mated  to  a  fly  with  yellow  color,  white  eyes, 
and  miniature  wings.  The  relative  location  of  these 
three  factors  is  shown  in  the  above  diagram  (Fig.  51, 


THE   MEXDELIAN   PRINCIPLES  OF  HEREDITY      99 


E,  F,  £,  H).  The  Fi  flies  gave  the  expected  re- 
sults. These  inbred  gave  the  following  F2  significant 
classes : 1 


GRL    YWM          GWM     YRL 

2089       1361  17  23 


GRM     YWL 

887         817 


GWL    YRM 
5-         0 


In  these  results  the  classes  where  single  crossing  over 
is  shown  are  GWM  (17)  and  YRL  (23)  (Fig.  51,  G, 
Gf)  and  GRM  (887)  and  YWL  (817)  (Fig.  51,  F,  F'). 

There  are  two  classes,  namely,  GWL  (5)  and  YRM 
(0)  (Fig.  51,  H,  Hr),  which  involve  double  crossing  over. 
In  order  that  they  may  take  place,  the  two  sex  chromo- 
somes in  the  female  must  break  twice  and  reunite 
between  the  factors  involved,  as  shown  in  the  diagram. 
Such  a  redistribution  of  the  parts  of  the  homologous 
chromosomes  would  be  expected  to  occur  rarely,  and 
the  small  number  of  double  crossovers  recorded  in 
the  results  is  in  accord  with  this  expectation. 

In  these  questions  of  linkage  we  have  considered 
some  of  the  most  recent  and  difficult  questions  in  the 
modern  study  of  heredity.  We  owe  to  Bateson  and 
his  collaborators  the  discovery  of  this  new  departure. 
In  plants  they  have  recorded  several  cases  of  linkage, 
and  other  authors,  notably  Correns,  Baur,  Emerson, 
East,  and  Trow  have  described  further  cases  of  the 
same  kind.  Bateson  has  offered  an  interpretation 
that  is  quite  different  from  the  one  that  I  have  here 
followed.  His  view  rests  on  the  assumption  that 
separation  of  factors  may  take  place  at  different  times, 
or  periods,  in  the  development  of  the  germinal  tissues. 

1  The  classes  omitted  are  those  that  do  not  bear  on  the  question 
in  hand. 


100  HEREDITY  AND   SEX 

In  a  word,  he  assumes  that  assortment  is  not  confined 
to  the  final  stages  in  the  ripening  of  the  germ-cells, 
but  may  take  place  at  any  time  in  the  germ-tract. 
It  seems  to  me,  however,  if  the  results  can  be  brought 
into  line  with  the  known  changes  that  take  place 
in  the  germ-cells  at  the  time  when  the  maternal  and 
paternal  chromosome  unite,  that  we  have  not  only 
a  simpler  method  of  dealing  with  these  questions, 
but  it  is  one  that  rests  on  a  mechanism  that  can  be 
studied  by  actual  observation.  Moreover,  on  purely 
a  priori  grounds  the  assumptions  that  I  have  made  seem 
much  simpler  and  more  tangible  than  the  assumptions 
of  " reduplication"  to  which  Bateson  resorts. 

But  leaving  these  more  theoretical  matters  aside, 
the  evidence  from  a  study  of  sex-linked  characters  shows 
in  the  clearest  manner  that  they,  while  following  Men- 
del's principle  of  segregation,  are  also  undeniably  asso- 
ciated with  the  mechajjism "  of  sex.  There  is  little 
doubt  that  sex  itself  is  inherited  in  much  the  same 
way,  since  we  can  explain  both  in  terms  of  the  same 
mechanism.  This  mechanism  is  the  behavior  of  the 
chromosomes  at  the  time  of  the  formation  of  the  germ- 
cells. 


CHAPTER  IV 

SECONDARY  SEXUAL   CHARACTERS  AND  THEIR   RELA- 
TION TO  DARWIN'S  THEORY  OF  SEXUAL  SELECTION 

IN  his  "Origin  of  Species "  Darwin  has  defined  Sexual 
Selection  as  depending  "on  a  struggle  between  the 
individuals  of  one  sex,  generally  the  males,  for  the 
possession  of  the  other  sex.  The  result  is  not  death 
to  the  unsuccessful  competitor,  but  few  or  no  offspring. 
Sexual  selection  is,  therefore,  less  rigorous  than  natural 
selection.  Generally,  the  most  vigorous  males,  those 
which  are  best  fitted  for  their  places  in  nature,  will 
leave  most  progeny.  But  in  many  cases,  victory 
depends  not  so  much  on  general  vigor,  as  on  having 
special  weapons,  confined  to  the  male  sex.  A  hornless 
stag  or  spurless  cock  would  have  a  poor  chance  of  leav- 
ing numerous  offspring.  Sexual  selection,  by  always 
allowing  the  victor  to  breed,  might  surely  give  indomi- 
table courage,  length  to  the  spur,  and  strength  to  the 
wing  to  strike  in  the  spurred  leg,  in  nearly  the  same 
manner  as  does  the  brutal  cock-fighter  by  the  careful 
selection  of  his  best  cocks." 

Darwin  continues:  "Amongst  birds,  the  contest 
is  often  of  a  more  peaceful  character.  All  those  who 
have  attended  to  the  subject,  believe  that  there  is  the 
severest  rivalry  between  the  males  of  many  species 
to  attract,  by  singing,  the  females.  The  rock-thrush 
of  Guiana,  birds  of  paradise,  and  some  others,  con- 

101 


102  HEREDITY  AND  SEX 


gregate,  and  successive  males  display,  with  the  most 
elaborate  care,  and  show  off  in  the  best  manner,  their 
gorgeous  plumage;  they  likewise  perform  strange 
antics  before  the  females,  which,  standing  by  as  spec- 
tators, at  last  choose  the  most  attractive  partner/' 

Here  we  have  two  different  pictures,  each  of  which 
attempts  to  give  an  account  of  how  certain  differences 
between  the  sexes  have  arisen  —  differences  that  we 
call  " secondary  sexual  characters." 

On  the  one  hand  we  deal  with  a  contest  between 
the  males;  on  the  other  with  choice  by  the  female. 
The  modus  operandi  is  also  different.  After  battle 
the  successful  male  takes  his  pick  of  the  females.  If 
the  scheme  is  to  work,  he  must  choose  one  that  will 
leave  the  most  offspring. 

On  the  other  hand,  we  have  the  tourney  of  love. 
The  males  "show  off";  the  females  stand  by  spell- 
bound and  "at  last  choose  the  most  attractive  partner." 

Now,  concerning  this  display  of  the  males,  I  beg 
leave  to  quote  a  paragraph  from  Wallace's  "Natural 
Selection  and  Tropical  Nature"  : 

"It  is  a  well-known  fact  that  when  male  birds  possess 
any  unusual  ornaments,  they  take  such  positions  or 
perform  such  evolutions  as  to  exhibit  them  to  the  best 
advantage  while  endeavoring  to  attract  or  charm 
the  females,  or  in  rivalry  with  other  males.  It  is 
therefore  probable  that  the  wonderfully  varied  decora- 
tions of  humming-birds,  whether  burnished  breast- 
shields,  resplendent  tail,  crested  head,  or  glittering 
back,  are  thus  exhibited  ;  but  almost  the  only  actual  ob- 
servation of  this  kind  is  that  of  Mr.  Belt,  who  describes 
how  two  males  of  the  Florisuga  mellivora  displayed 


SECONDARY  SEXUAL  CHARACTERS  103 

their  ornaments  before  a  female  bird.  One  would 
shoot  up  like  a  rocket,  then,  suddenly  expanding  the 
snow-white  tail  like  an  inverted  parachute,  slowly 
descend  in  front  of  her,  turning  around  gradually  to 
show  off  both  back  and  front.  The  expanded  white 
tail  covered  more  space  than  all  the  rest  of  the  bird,  and 
was  evidently  the  grand  feature  of  the  performance. 
Whilst  one  was  descending  the  other  would  shoot 
up  and  come  slowly  down  expanded." 

There  is  just  a  suspicion  in  my  mind  that  these  males 
were  otherwise  engaged,  for  while  I  know  nothing 
about  the  habits  of  these  humming  birds  I  find  on  the 
next  page  of  "  Tropical  Nature  "  this  statement : 

"Mr.  Gosse  also  remarks:  'All  the  humming- 
birds have  more  or  less  the  habit,  when  in  flight,  of 
pausing  in  the  air  and  throwing  the  body  and  tail  into 
rapid  and  odd  contortions.  This  is  most  observable 
in  Polytmus,  from  the  effect  that  such  motions  have 
on  the  long  feathers  of  the  tail.  That  the  object  of 
these  quick  turns  is  the  capture  of  insects,  I  am  sure, 
having  watched  one  thus  engaged.' ' 

If  what  I  have  just  said  implies  that  I  take  a  light- 
hearted  or  even  facetious  attitude  toward  Darwin's 
theory,  I  trust  that  my  position  will  not  be  misunder- 
stood. Darwin  brought  together  in  his  book  on  the 
" Descent  of  Man"  a  mass  of  interesting  observations 
for  which  he  suggested  a  new  theory.  No  one  can 
read  his  wonderful  book  without  the  keenest  interest, 
or  leave  it  without  high  admiration  for  the  thorough- 
ness with  which  the  subject  is  treated  ;  for  the  ingenuity 
and  skill  with  which  the  theory  is  applied  to  the  facts, 
and,  above  all,  admiration  for  the  moderation,  modesty, 


104  HEREDITY  AND   SEX 

and  honesty  with  which  objections  to  the  theory  are 
considered. 

I  will  let  no  one  admire  Darwin  more  than  I  admire 
Darwin.  But  while  affection  and  respect  and  honor 
are  the  finest  fruits  of  our  relation  to  each  other,  we 
cannot  let  our  admiration  for  the  man  and  an  ever 
ready  recognition  of  what  he  has  done  for  you  and  for 
me  prejudice  us  one  whit  in  favor  of  any  scientific 
theory  that  he  proposes.  For  in  Science  there  is  no 
authority  !  We  should  of  course  give  serious  considera- 
tion to  any  theory  proposed  by  a  man  of  such  wide  expe- 
rience and  trained  judgment  as  Darwin  ;  but  he  himself, 
who  broke  all  the  traditions  of  his  race,  would  be  the 
first  to  disclaim  the  value  of  evidence  accepted  on 
authority. 

From  the  definition  of  sexual  selection  with  which 
we  started  it  may  be  said  that  Competition  and  Courtship 
stand  for  the  two  ways  in  which  Darwin  supposes 
the  secondary  sexual  characters  to  have  arisen. 

Competition  amongst  the  males  is  only  a  form  of  nat- 
ural selection,  as  Darwin  himself  recognized  (if  we  leave 
out  of  account  the  further  assumption  that  the  victor 
chooses  his  spoils).  We  may  dismiss  this  side  of  the 
problem  as  belonging  to  the  larger  field  of  natural  selec- 
tion, and  give  our  attention  mainly  to  those  secondary 
sexual  characters  that  Darwin  supposes  to  have  arisen 
by  the  female  choosing  the  more  ornamented  suitor. 

I  shall  first  bring  forward  some  of  the  more  striking 
examples  of  secondary  sexual  characters  in  the  animal 
kingdom.  These  characters  are  confined  almost  ex- 
clusively to  three  great  groups  of  animals  —  Insects, 


SECONDARY  SEXUAL  CHARACTERS 


105 


Spiders,  and  Vertebrates.  There  are  a  few  scattered 
instances  found  in  other  groups,  but  they  are  rare. 
In  the  lowest  groups  they  are  entirely  absent,  and  are 


FIG.  52.  —  Four  species  of  beetles  in  which  the  male  (to  the  left)  has  horns 
which  are  absent  in  the  female  (to  the  right).     (After  Darwin.) 

not  found  at  all  in  plants;  or  rather,  if  character- 
istic differences  exist  in  plants,  they  are  not  called  by 
this  name  —  for  plants  cannot  see  or  move  and  there- 
fore cannot  court  each  other. 


106 


HEREDITY   AND   SEX 


In  fact,  sight  in  the  sense  of  forming  visual  pictures 
can  occur  only  when  eyes  are  well  developed.     This 


FIG.  53.  —  Male  (to  left)  with  long  eye  stalks  and  female  (to  right)  of 
a  fly,  Achia  longividens.     (After  Wood.) 

may  be  taken  to  score  a  point  in  favor  of  Darwin's 
hypothesis. 

In  the  group  of  insects  the  most  noticeable  differences 
occur  in  the  butterflies  and  moths,  and  in  flies.  A 
few  cases  are  found  in  the  beetles  and  bugs.  The 
male  cicada's  shrill  call  is  supposed  to  attract  the 


FIG.  54.  —  Male  to  left  with  horns  and  female  to  right  without  horns  of  a 
fly,  Elaphomyia.     (After  Wood.) 

females.     The  males  of  certain  beetles  have  horns  - 
the  female  lacks  them  (Fig.  52). 

In  a  genus  of  flies  the  eyes  are  stalked,  and  the 


SECONDARY  SEXUAL  CHARACTERS  107 

eyes  of  the  male  have  stalks  longer  than  those  of 
the  female  (Fig.  53).  In  another  genus  of  flies  there 
are  horns  on  the  head  like  the  antlers  of  the  stag 
(Fig.  54). 

'In  the  spiders  the  adult  males  are  sometimes  very 
small  in  comparison  with  the  females  (Fig.  55).  The 
size  difference  may  be  regarded  as  a  secondary  sexual 


FIG.  55.  —  Male  (to  left)  and  female  (to  right)  of  a  spider,  Argiope  aurelia. 
(From  "  Cambridge  Natural  History.") 

character.  Darwin  points  out,  since  the  male  is  some- 
times devoured  by  the  female  (if  his  attentions  are 
not  desired),  that  his  small  size  may  be  an  adaptation 
in  order  that  he  may  more  readily  escape.  But  the 
point  may  be  raised  as  to  whether  he  is  small  in  order 
to  escape ;  or  whether  he  is  eaten  because  he  is  small. 
In  one  of  our  native  spiders,  Habrocestum  splendida, 
the  adult  males  and  females  are  conspicuously  different 


108  HEREDITY  AND   SEX 

in  color  —  the  male  more  highly  colored  than  the 
female.  In  another  native  species,  Maevia  vittata, 
there  are  two  kinds  of  males,  both  colored  differently 
from  the  female. 

Passing  over  the  groups  of  fishes  and  reptiles  in 
which  some  striking  cases  of  differences  between  the 
sexes  occur,  we  come  to  the  birds,  where  we  find  the 
best  examples  of  secondary  sexual  characters. 


FIG.  56.  —  Superb  bird  of  paradise. 
(After  Elliot.) 

In  the  white-booted  humming  bird  (Fig.  14)  two 
of  the  tail  feathers  of  the  male  are  drawn  out,  their 
shafts  denuded  of  the  vanes  except  at  the  tip  where 
the  feather  ends  in  a  broad  expansion. 

In  the  great  bird  of  paradise,  of  the  Aru  Islands  (Fig. 
13),  the  male  has  wonderful  plumes  arising  from  the 
sides  that  can  be  erected  to  produce  a  gorgeous  display. 


SECONDARY  SEXUAL  CHARACTERS  109 

The  female  is  modestly  clothed.  In  the  male  of  the 
superb  bird  of  paradise  (Fig.  56),  the  mantle  behind 
the  neck,  when  erected,  forms  a  striking  ornament ; 
and  on  the  breast  there  is  a  brilliant  metallic  shield. 
In  the  six-shafted  bird  of  paradise  (Fig.  57)  the 
male  has  on  its  head  six  feathers  with  wiry  shafts, 


FIG.  57.  —  Six-shafted  bird  of  paradise. 
(After  Elliot.) 

ornaments  that  occur  in  no  other  birds.  In  the  king 
bird  of  paradise  there  are  remarkable  fans  at  the 
sides  of  the  body  of  the  male  that  can  be  expanded. 
The  feathers  of  the  fan  are  emerald-tipped.  The 
two  middle  feathers  of  the  tail  are  drawn  out  into 
"wires"  with  a  green  web  at  one  side  of  the  tip. 

In  mammals,  secondary  sexual  differences  are  very 


110 


HEREDITY  AND   SEX 


common,  although  startling  differences  in  color  are 
rather  rare.  In  the  male  the  coat  of  fur  is  often  darker 
than  that  of  the  female. 

In  many  deer  the  antlers  are  present  in  the  male 
alone.  In  Steller's  sea-lion  the  male  is  much  larger 
and  stronger  than  the  female.  In  a  race  of  the  Asiatic 
elephant  the  male  has  tusks  much  larger  than  those 
of  the  female. 

If  we  fix  our  attention  exclusively  on  these  remarkable 


FIG.  58.  —  Wilson's  phalarope,  female  (in  center) ,  male  (to  right  and 
behind).  A  bird  in  winter  plumage  is  at  the  left.  (From  Eaton,  "Birds  of 
New  York.") 

cases  where  differences  between  the  sexes  exist,  we 
get  a  one-sided  impression  of  the  development  of 
ornamentation  and  color  differences  in  animals.  We 
must  not  forget  that  in  many  cases  males  and  females 
are  both  highly  colored  and  exactly  alike.  We  forget 
the  parrots,  the  cockatoos,  the  kingfishers,  the  crowned 
pigeons,  toucans,  lories,  and  some  of  the  starlings; 
the  " brilliant  todies"  and  the  " sluggish  jacamars" 
whose  brilliant  metallic  golden-green  breasts  rival 


SECONDARY  SEXUAL  CHARACTERS 


111 


those  of  the  humming-birds;  we  forget  the  zebras, 
the  leopards ;  the  iridescent  interiors  of  the  shells  of 
many  mollusks ;  the  bright  reds  and  purples  of  starfish, 
worms,  corals,  sea  anemones,  the  red,  yellow,  and  green 
sponges,  and  the  kaleidoscopic  effect  of  the  microscopic 
radiolarians  ;  —  a  brilliant  array  of  color. 


FIG.  59.  —  (A),  female  of  a  copepod,  Calocalanus  plumulosus.     (B),  a  female 
of  Calocalanus  parous.     (C),  male  of  last.     (After  Giesbrecht.) 

In  the  egret  both  males  and  females  have  remark- 
able nuptial  plumes,  which,  had  they  been  present  in 
one  sex  alone,  would  have  been  classified  as  secondary 
sexual  characters.  It  does  not  appear  that  selection 
had  anything  to  do  with  their  creation. 

Our  common  screech  owl  exists  in  two  colored  types 
sharply  separated.  No  one  is  likely  to  ascribe  these 
differences  to  sexual  selection,  yet  if  one  sex  had  been 


112  HEREDITY  AND   SEX 

red  and  the  other  gray,  the  difference  would  have  been 
put  down  to  such  selection.  There  are  also  cases  like 
the  phalarope,  shown  in  Fig.  58,  where  the  female  is 
more  highly  ornamented  than  the  male.  In  fact,  for 
these  cases,  Darwin  supposed  that  the  males  select 
the  females ;  and  in  support  of  this  view  he  points  out 
that  the  females  are  more  active,  while  the  male  con- 
cerns himself  with  the  brooding  of  the  eggs.  In  some 
of  the  marine  copepods  female  ornamentation  is  car- 
ried to  even  a  higher  point.  In  Calocalanus  plumosus 
the  female  has  one  of  the  tail  setae  drawn  out  into  a  long 
feather-like  structure  (Fig.  59).  In  another  species, 
C.  parva,  all  eight  setae  of  the  tail  of  the  female  are 
feather-like  (Fig.  59,  B),  while  the  male  (Fig.  59,  C) 
lacks  entirely  these  " ornaments." 

In  some  butterflies  also,  two,  three,  or  more  types  of 
females  are  known,  but  only  one  male  type.  I  shall 
have  occasion  later  to  consider  this  case. 

COURTSHIP 

The  theory  of  sexual  selection  hinges  in  the  first 
place  on  whether  the  female  chooses  amongst  her 
suitors. 

It  has  been  objected  that  the  theory  is  anthropo- 
morphic —  it  ascribes  to  beetles,  butterflies,  and  birds 
the  highly  developed  esthetic  sense  of  man.  It  has 
been  objected  that  the  theory  leaves  unexplained  the 
development  of  this  esthetic  sense  itself,  for  unless  the 
female  kept  in  advance  of  the  male  it  is  not  self-evident 
why  she  should  go  on  selecting  the  more  highly  orna- 
mented. If  she  has  advanced  esthetically,  what  has 
brought  it  about?  In  answer  to  this  last  question 


SECONDARY  SEXUAL  CHARACTERS  113 

Allen  suggests  that  if  the  word  conspicuousness  is  sub- 
stituted for  the  word  beauty,  the  objection  may  to  some 
extent  be  met.  The  more  conspicuous  male  would  be 
more  likely  to  attract  attention  and  be  selected. 

It  has  been  pointed  out  that  there  is  more  than  a 
suspicion  that  the  contests  of  the  males  for  the  females 
are  sham  affairs.  They  are  like  certain  duels.  There 
is  seldom  any  one  hurt.  There  are  very  few  records  of 
injured  males,  but  many  accounts  of  tremendous 
battles.  And  he  who  fights  and  runs  away  will  live 
to  mate  another  day. 

It  is  clear,  I  think,  that  the  case  against  the  theory 
must  rest  its  claims  on  actual  evidence  rather  than  on  ar- 
guments or  poetry  pro  or  con.  Darwin  admitted  that 
the  evidence  was  meager.  Since  his  time  something 
more  has  been  done.  Let  us  consider  some  of  this  new 
evidence. 

It  will  be  conceded,  I  think,  that  Alfred  Wallace, 
through  his  wide  experience  with  animals  in  their 
native  haunts,  is  in  a  position  to  give  weighty  evidence 
concerning  the  behavior  of  animals.  He  was  with 
Darwin  a  co-discoverer  of  the  theory  of  Natural  Se- 
lection and  cannot  be  supposed  to  be  prejudiced  against 
the  selection  principle.  Yet  Wallace  has  from  the 
beginning  strongly  opposed  the  theory  of  sexual  se- 
lection. Let  me  quote  him  : 

Referring  to  Darwin's  theory  of  Sexual  Selection  - 

"I  have  long  held  this  portion  of  Darwin's  theory  to 
be  erroneous  —  and  have  held  that  the  primary  cause 
of  sexual  diversity  of  color  was  the  need  of  protection, 
repressing  in  the  female  those  bright  colors  which  are 
normally  produced  in  both  sexes  by  general  laws." 


114  HEREDITY  AND   SEX 

Again,  Wallace  says:  "To  conscious  sexual  selec- 
tion—  that  is,  the  actual  choice  by  the  females  of  the 
more  brilliantly  colored  males  or  the  rejection  of  those 
less  gaily  colored  —  I  believe  very  little  if  any  effect 
is  directly  due.  It  is  undoubtedly  proved  that  in 
birds  the  females  do  sometimes  exert  a  choice ;  but 
the  evidence  of  this  fact,  collected  by  Mr.  Darwin 
('Descent  of  Man/  chap,  xiv),  does  not  prove  that  color 
determines  that  choice,  while  much  of  the  strongest 
evidence  is  directly  opposed  to  this  view.7' 

Again,  Wallace  says:  "Amid  the  copious  mass  of 
facts  and  opinions  collected  by  Mr.  Darwin  as  to  the 
display  of  color  and  ornaments  by  the  male  birds,  there 
is  a  total  absence  of  any  evidence  that  the  females,  as 
a  rule,  admire  or  even  notice  this  display.  The  hen, 
the  turkey,  and  the  peafowl  go  on  feeding,  while  the 
male  is  displaying  his  finery;  and  there  is  reason  to 
believe  that  it  is  his  persistency  and  energy  rather  than 
his  beauty  which  wins  the  day." 

Hudson,  who  has  studied  the  habits  of  birds  in  the 
field,  asks  some  very  pertinent  questions  in  connec- 
tion with  their  performances  of  different  kinds.  ' i  What 
relation  to  the  passion  of  love  and  to  the  business  of 
courtship  have  these  dancing  and  vocal  performances 
in  nine  cases  out  of  ten  ?  In  such  cases,  for  instance, 
as  that  of  the  scissor-tail  tyrant-bird,  and  its  pyro- 
technic displays,  when  a  number  of  couples  leave  their 
nests  containing  eggs  and  young  to  join  in  a  wild  aerial 
dance ;  the  mad  exhibition  of  grouped  wings ;  the 
triplet  dances  of  the  spur-winged  lapwing,  to  perform 
which  two  birds  already  mated  are  compelled  to  call 
in  a  third  bird  to  complete  the  set;  the  harmonious 


SECONDARY  SEXUAL  CHARACTERS     115 

duets  of  the  oven-birds  and  the  duets  and  choruses  of 
nearly  all  the  wood-hewers,  and  the  wing-slapping 
aerial  displays  of  the  whistling  widgeons,  —  will  it  be 
seriously  contended  that  the  female  of  this  species 
makes  choice  of  the  male  able  to  administer  the  most 
vigorous  and  artistic  slaps  ?" 

He  continues:  "How  unfair  the  argument  is, 
based  on  these  carefully  selected  cases,  gathered  from 
all  regions  of  the  globe,  and  often  not  properly  reported, 
is  seen  when  we  turn  to  the  book  of  nature  and  closely 
consider  the  habits  and  actions  of  all  the  species  in- 
habiting any  one  district."  Hudson  concludes  that  he 
is  convinced  that  anybody  who  will  note  the  actions  of 
animals  for  himself  will  reach  the  conviction,  that 
"conscious  sexual  selection  on  the  part  of  the  female 
is  not  the  cause  of  music  and  dancing  performances  in 
birds,  nor  of  the  brighter  colors  and  ornaments  that 
distinguish  the  male." 

In  the  spiders  Mr.  and  Mrs.  Peckham  have  described 
in  detail  the  courtship  of  the  males.  They  believe 
that  his  antics  are  specifically  intended  to  attract  the 
female.  They  point  out  that  his  contortions  are  of 
such  a  sort  that  his  brightest  spots  are  turned  toward 
the  female.  But,  as  he  makes  in  any  case  a  hundred 
twists  and  turns,  there  is  some  danger  of  misinterpret- 
ing his  poses.  Montgomery,  who  has  studied  spiders 
of  other  groups,  reaches  the  conclusion  that  here  the 
male  is  contorted  through  fear  of  the  female.  The  male 
goes  through  some  of  the  same  turns  if  approached  by 
another  male.  The  courtship  of  the  male  spider  is, 
he  thinks,  a  motley  of  fear,  desire,  and  general 
excitement. 


116  HEREDITY  AND   SEX 

The  evidence  that  the  Peckhams  have  given,  even  if 
taken  to  mean  that  the  motions  of  the  male  attract 
the  attention  of  the  female,  —  and  I  can  see  no  reason 
why  this  may  not  be  the  case, — fails  nevertheless  to  show 
that  the  female  selects,  when  she  has  a  chance,  the  more 
highly  colored  male. 

Mayer,  and  Mayer  and  Soule  have  made  many  ex- 
periments with  moths.  The  moth  promethea,  Callo- 


FIG.  60.  —  Above,  Callosamia  promethia,  male  to  left,  female  to  right. 
Below  Porthetria  dispar,  male  to  left,  female  to  right. 

samia  promethea,  is  distinctly  sexually  dimorphic,  as 
shown  in  Fig.  60.  Mayer's  experiments  show  that  the 
male  finds  the  female  entirely  by  the  sense  of  smell. 
The  wings  of  some  300  males  were  painted  with  scarlet 
or  green.  They  mated  as  often  as  did  the  normal  male 
with  which  they  competed. 

Where  the  wings  of  males  were  stuck  on  the  female 
in  place  of  her  own  wings,  no  disturbance  in  the  mating 
was  observed.  Conversely,  normal  females  accepted 


SECONDARY  SEXUAL  CHARACTERS  117 

males  with  female  wings  as  readily  as  they  accepted 
normal  males. 

In  the  gipsy  moth  (Porthetria  dispar),  the  male  is 
brown  and  the  female  white  (Fig.  60).  Here  again 
it  was  found  that  the  males  are  guided  solely'  by  the 
odor  of  the  female. 

The  silkworm  moth  is  also  sexually  dimorphic.  Kel- 
logg has  shown  that  males  with  blackened  eyes  find  a 
female  with  as  much  precision  as  does  a  moth  with 
normal  eyes. 

If  the  antennae  are  cut  off,  however,  the  male  can  not 
find  the  female  unless  by  accident  he  .touches  her.  He 
then  mates.  The  female  has  scent  glands  whose  odor 
excites  the  male  with  normal  antennae  even  at  some  dis- 
tance. Chemotaxis  and  contact  are  the  active  agents 
in  mating.  The  eyes  do  little  or  nothing. 

Andrews  has  found  that  touch  determines  mating  in 
the  crayfish.  Pearse  has  obtained  similar  results. 
Chidester  has  shown  the  same  thing  for  crabs.  Holmes 
found  this  kind  of  behavior  in  Amphipoda.  Fielde  and 
Wheeler  have  also  found  that  in  ants  sex-discrimination 
is  through  smell  or  by  what  Forel  calls  contact-odors. 

Montgomery  and  Porter  recognize  touch  as  the  most 
important  factor  in  mating  in  spiders.  Petrunke- 
witsch  has  shown  that  in  the  hunting  spider  vision  also 
helps  the  sexes  to  find  each  other.  Tower  has  found 
that  contact  or  odor  rather  than  sight  is  the  important 
condition  in  mating  in  leptinotarsa. 

I  am  able  to  give  the  unpublished  results  of  A.  H. 
Sturtevant  on  the  mating  of  the  fruit  fly,  drosophila. 
The  male  carries  on  an  elaborate  courtship  in  the 
sense  that  he  circles  around  the  female,  throws  out  one 


118  HEREDITY  AND   SEX 

wing,  then  the  other,  and  shows  other  signs  of  excite- 
ment. The  male  has  sex  combs  on  his  fore  legs,  the 
female  lacks  them.  Lutz  cut  them  off  and  gave  the 
female  a  choice  between  such  a  male  and  a  normal 
male.  One  was  chosen  as  often  as  the  other.  The 
wings  of  the  male  and  female  are  wonderfully  irides- 
cent. Sturtevant  cut  off  the  wings  of  a  male  and 
matched  him  against  a  normal  male.  The  female 
showed  no  marked  preference.  The  converse  experi- 
ment, when  a  clipped  female  competed  with  a  normal 
female,  showed  no  selection  on  the  part  of  the  males. 

If  instead  of  allowing  two  males  (a  normal  and  a 
clipped)  to  compete  for  one  female,  a  female  is  given  to 
each  male  separately,  and  the  interval  before  mating  is 
noted,  it  is  found  that  on  an  average  this  interval  is  18 
minutes  for  the  normal  and  40  minutes  for  the  clipped.  If 
any  such  difference  existed  in  the  first  case,  when  the  two 
males  were  competing,  we  should  expect  a  much  greater 
selection  in  favor  of  the  normal  male  than  was  actually 
found.  This  would  seem  to  mean  that  the  female  is 
more  quickly  aroused  by  the  normal  male,  and  hence 
when  both  males  are  present  she  will  accept  the  clipped 
male  more  quickly  than  when  he  alone  is  present.  This 
suggests  that  normal  courtship  precipitates  copulation. 

In  the  following  experiments  the  female  was  offered 
a  choice  between  a  new  type  (mutant)  with  white  eyes 
and  a  normal  male.  Conversely,  the  white-eyed  fe- 
male had  a  like  alternative.  The  evidence  shows  that 
the  more  vigorous  male  —  the  red-eyed  male  —  is 
more  successful. 

Since  vision  itself  is  here  involved,  for  the  white- 
eyed  flies  are  probably  partly  blind,  the  observations 


SECONDARY  SEXUAL  CHARACTERS 


119 


RED   VERSUS   WHITE  EYES. 


GRAY  VERSUS  YELLOW  COLOR. 


Rede? 

|  Red  9      -  54 
{  White  9-82 

Gray  cf 

j  Gray  9      -  25 
\  Yellow  9-31 

White  J 

j  Red  9      -  40 
|  White  9-93 

Yellow  d" 

f  Gray  9      -  12 
{  Yellow  9-30 

Red  9 

f  Red  d"      -  53 
\  White  cf  -  14 

Gray  9 

{  Gray  cf      -  60 
{  Yellow  cf  -  12 

White  9 

{  Red  cf      -  62 
|  White  d"  -  19 

Yellow  9 

J  Gray  cf      -  25 
[  Yellow  cf   -   8 

NORMAL  VERSUS   CLIPPED 

GRAY-WHITE    VERSUS    YELLOW- 

WINGS. 

WHITE. 

Normal   J 

|  Clipped  cf  -  51 
|  Normal  cf  -  67 

Gray  cf 

f  Gray  9      -  11 
(  Yellow  9-4 

Normal    -? 

{  Clipped  9-27 
|  Normal  9-25 

Gray  9 

f  Gray  cf    -  21 
(  Yellow  cf  -    3 

were  repeated  with  a  new  type  that  had  yellow  wings. 
The  gray  male  is  more  successful  and  the  yellow  females 
less  resistant.  The  results  are  in  accord  with  the  as- 
sumption that  greater  vigor  is  an  important  factor 
in  success. 

The  following  mating  bears  on  this  point.  Stur- 
tevant  used  in  competition  a  red-  and  a  vermilion- 
eyed  male.  The  latter  seems  as  vigorous  as  is  the 
red-eyed  type.  The  results  were  : 


Red 


Red  $ 
Vermilion  $ 


11 

14 


showing   that   the   red-eyed   male   has   no   advantage 
when  the  males  are  equally  vigorous. 

This  evidence,  taken  as  a  whole,  seems  to  me  to  show 
with  some  probability  that  sight  plays  a  minor  role  in 


120  HEREDITY  AND   SEX 

courtship.  It  is  so  inferior  to  vigor,  to  the  sense  of 
smell  and  to  touch  in  the  lower  animals  at  least,  that 
it  is  very  questionable  whether  it  has  had  anything 
more  to  do  with  mating  than  helping  the  sexes  find 
each  other. 

VIGOR   AND    SECONDARY    SEXUAL    CHARACTERS 

We  have  seen  that  Darwin  himself  has  stated  ex- 
plicitly that  unless  the  secondary  sexual  characters 
are  associated  with  greater  vigor,  or  productivity, 
nothing  can  be  accomplished. 

It  will  be  recalled  that  Wallace,  who  disbelieved  in 
Darwin's  theory  of  sexual  selection,  attempted  to  ac- 
count for  the  appearance  of  secondary  sexual  characters 
on  the  ground  of  the  greater  vigor  of  the  male  (he 
sometimes  says  vitality  and  again  activity  of  the  male) 
at  the  breeding  season.  The  vigor  is  assumed  to  be 
associated  with  the  development  of  the  sex  glands 
at  this  time.  This  may  be  admitted,  but  whether  the 
vigor  is  the  result  of  the  sex  glands,  or  the  sex  glands  of 
the  vigor,  is  a  nice  point  that  I  shall  not  try  to  decide. 
It  may  appear  that  Wallace's  view  is  in  part  justified 
from  the  facts  that  we  have  examined.  But  I  do  not 
think  so.  In  the  first  place,  he  attempts  the  impos- 
sible task  of  explaining  the  outgrowths  and  colors  that 
appear  in  special  regions  by  the  local  activity  of  the 
muscles  (for  example)  in  those  regions.  The  facts 
before  us  do  not  support  any  such  interpretation.  The 
Peckhams  easily  overturn  his  argument,  as  applied  to 
spiders. 

Second,  in  birds,  to  which  Wallace  mainly  refers,  the 
sex  glands  of  the  male  do  not  affect  the  secondary 


SECONDARY  SEXUAL  CHARACTERS  121 

sexual  characters  of  the  male,  while  the  sex  glands  of  the 
female  suppress  these  characters. 

Wallace's  theory  leaves  out  of  account  the  hereditary 
factor  that  is  also  present  and  which  acts  quite  apart 
from  the  physiological  effects  of  the  sex  glands. 

Cunningham,  who  has  more  recently  written  on  the 
same  subject,  accepts  the  hormone  hypothesis  as  the 
basis  for  all  cases  of  secondary  sexual  characters. 
But  he  fails  to  make  good  his  view  when  it  is  applied 
to  insects,  for  reasons  that  we  shall  take  up  later.  He  is 
especially  concerned,  however,  in  the  attempt  to  make 
plausible  his  own  hypothesis  that  secondary  sexual 
characters  have  arisen  through  the  use  of  the  parts,  or 
through  special  nervous  or  blood  supplies  to  certain  lo- 
calities of  the  body  which  become  suffused  during  sexual 
excitement.  In  both  cases  he  thinks  the  increased  local 
activity  will  cause  the  cells  to  produce  hormones  that 
will  be  dispersed  throughout  the  body,  and  absorbed 
by  other  cells.  The  germ-cells  will  in  this  way  get 
their  share  and  carry  over  the  hormone  to  the  next 
generation. 

Cunningham  forgets  one  important  point.  If  these 
imaginary  hormones  can  get  out  of  cells  and  into  germ- 
cells,  they  can  get  out  of  the  germ-cells  again.  Hence 
in  the  long  period  of  embryonic  and  juvenile  existence 
through  which  the  individual  passes  before  the  second- 
ary sexual  characters  appear  they  would  surely  be  lost 
from  the  body  like  any  other  ordinary  hormone. 

CONTINUOUS   VARIATION    AS    A   BASIS   FOR    SELECTION 

And  now  let  us  turn  to  an  entirely  different  aspect  of 
the  matter.  What  could  selection  do,  admitting  that 
^election  may  take  place.  For  fifty  years  it  has  been 


122 


HEREDITY  AND   SEX 


n 


A-E 


FIG.  61.  —  I.  Diagram  of  five  pure  lines  of  beans  (A,  B,  C,  D,  and  E) 
and  a  population  formed  by  their  union,  A-E.  II.  Diagrams  illustrating  a 
pure  line  of  beans  and  two  new  biotypes  derived  from  it.  The  upper 
diagram  indicates  the  original  biotype ;  the  second  and  third  diagram  in- 
dicate the  elongated  (narrower)  and  shorter  (broader)  type  of  beans.  X 
indicates  the  average  class  of  the  original  biotype.  (After  Johannsen.) 


SECONDARY  SEXUAL  CHARACTERS 


123 


taken  for  granted  that  by  selecting  a  particular  kind 
of  individual  the  species  will  move  in  the  direction 
of  selection. 

A  few  examples  will  bring  the  matter  before  us.  If 
we  take  a  peck  of  beans  and  put  all  of  those  of  one  size 
in  one  cylinder  and  those  of  other  sizes  in  other  cyl- 
inders, and  place  the  cylinders  in  a  row,  we  get  a  result 
like  that  in  Fig.  61,  A-E.  If  we  imagine  a  line  joining 


26 


+36 


FIG.  62.  —  The  normal  binomial  curve  or  the  "ideal  curve"  of  distribu- 
tion. At  the  base  line,  the  directions  from  the  average  value  (o)  are 
indicated  with  the  standard  deviation  (<r)  as  unity.  (After  Johannsen.) 

the  tops  of  the  beans,  the  line  gives  a  curve  like  that 
shown  in  Fig.  62.  This  is  known  as  the  curve  of  prob- 
ability. The  curve  can  be,  of  course,  most  readily 
made  by  making  the  measurements  directly.  Most 
individuals  of  such  a  population  will  have  the  charac- 
ter developed  to  the  degree  represented  by  the  highest 
point  in  the  curve.  Now  if  two  individuals  standing 
at  one  side  (let  us  say  with  the  character  in  question 
better  developed  than  the  average)  become  the  parent 


124 


HEREDITY   AND   SEX 


of  the  next  generation,  their  offspring  will  make  a  new 
curve  that  has  moved,  so  to  speak,  in  the  direction  of 
selection  (Fig.  63). 

If  again  two  more  extreme  individuals  are  selected, 
another  step  is  taken.  The  process  is  assumed  to  go 
on  as  long  as  the  selection  process  is  maintained. 

So  the  matter  stood  until  a  Danish  botanist,  Johann- 
sen,  set  seriously  to  work  to  test  the  validity  of  the 
assumption,  using  a  race  of  garden  beans  for  his  meas- 
urements. He  discovered  in  the  first  place  that  popu- 


FIG.  63.  —  Schematic  representation  of  the  type-shifting  effect  of  selec- 
tion from  the  point  of  view  of  Galton's  regression  theory.  The  *  marks  the 
point  on  the  curves  of  A,  Ai,  Az  from  which  the  selection  is  supposed  to  be 
made.  (Goldschmidt.) 

lations  are  made  up  of  a  number  of  races  or  "pure 
lines."  When  we  select  in  such  a  population  we  sort 
out  and  separate  its  constituent  races,  and  sooner 
or  later  under  favorable  conditions  can  get  a  pure 
race.  Selection  has  created  nothing  new;  it  has 
picked  out  a  particular  preexisting  race  from  a  mixed 
population. 

Johannsen  has  shown  that  within  a  pure  line  selec- 
tion produces  no  effect,  since  the  offspring  form  the 
same  group  with  the  same  mode  as  the  group  from  which 
the  parents  came.  The  variability  within  the  pure 
lines  is  generally  ascribed  to  environmental  influences 


SECONDARY  SEXUAL   CHARACTERS  125 

which  are  recurrent  in  each  generation.  The  germ- 
plasm  is  homogeneous  for  all  members  of  the  pure  line, 
while  in  a  mixed  population  the  germ-plasm  is  not  the 
same  for  all  individuals. 

Darwin  himself  saw  this  to  some  extent,  for  he  has 
repeatedly  pointed  out  that  selection  depends  on  the 
materials  offered  to  it  by  variation ;  that  in  itself  it 
can  produce  nothing.  Yet  from  Darwin  to  Johannsen 
the  teaching  of  the  post-Darwinians  has  been  such  as  to 
lead  most  people  to  believe  that  selection  is  a  causative 
or  creative  principle  that  will  explain  the  progressive 
development  of  animals  and  plants. 

DISCONTINUOUS    VARIATION    OR    MUTATION    AS   A   BASIS 
FOR    SELECTION 

The  second  great  movement  since  Darwin  has  been 
to  show  that  hereditary  variations  do  not  give  a  con- 
tinuous series  but  a  discontinuous  one.  Bateson  and 
De  Vries  brought  forward  some  twelve  years  ago  evi- 
dence, in  favor  of  this  view,  that  has  gone  on  increasing 
in  volume  at  an  amazing  rate. 

I  cannot  attempt  to  discuss  this  evidence  here,  but 
I  may  point  out  the  bearing  of  the  new  point  of  view 
on  the  meaning  of  secondary  sexual  characters. 

In  a  number  of  butterflies  there  occur  two  or  three 
or  even  more  different  kinds  of  females.  One  of  the  most 
remarkable  cases  of  the  kind  is  that  of  Papilio  polytes 
that  lives  in  India  and  Ceylon.  It  has  a  single  male 
type  and  three  types  of  females  (Fig.  64). 

Wallace,  who  first  observed  that  the  three  types  of 
female  belong  to  one  male  type,  argued  that  two 
of  these  three  types  owe  their  origin  to  their  resem- 


126 


HEREDITY  AND   SEX 


blance  to  butterflies  of  other  species  that  are  protected, 
namely,    Papilio  aristolochia    and    P.  hector.       These 


FIG.  64.  —  Papilio  polytes  ;  male  (upper  left)  and  three  types  of  female 
(to  right).  The  "models,"  which  two  of  these  females  are  supposed  to 
"mimic,"  are  shown  to  their  left.  (After  Punnett.) 

two  feed  on  the  poisonous  plant  aristolochia  and  are 
said  to  be  unpalatable.  The  two  aberrant  types  of 
P.  polytes  bearing  a  close  resemblance  to  these  two 


SECONDARY  SEXUAL  CHARACTERS  127 

species  have  been  dubbed  the  hector  form  and  the 
aristolochia  form. 

Wallace,  and  those  who  adhere  to  the  same  view, 
believe  that  the  resemblance  of  the  model  and  the 
mimic  has  come  about  through  the  accumulation  of 
minute  variations  which  have  survived  as  a  result  of 
their  advantages.  In  a  word,  the  process  of  natural 
selection  is  assumed  to  have  gradually  brought  about 
the  evolution  of  these  two  new  types  of  females. 

This  case  has  been  recently  examined  by  Punnett. 

Punnett  says  that  while  in  cabinet  specimens  the 
resemblance  between  the  model  and  the  mimic  is  re- 
markably close,  yet  in  the  living  animals,  with  their 
wings  spread  out,  the  resemblance  is  less  marked,  espe- 
cially the  resemblance  between  the  hector  model  and 
the  polytes  mimic.  At  a  distance  of  a  few  yards  the 
difference  between  the  two  is  easily  seen. 

When  flying  the  differences  are  very  apparent.  ' '  The 
mode  of  flight  of  P.  polytes  is  similar  for  all  three  forms, 
and  is  totally  distinct  from  that  of  P.  hector  and  P. 
aristolochia."  In  flight  the  latter  pursue  an  even 
course,  while  the  polytes  form  follow  a  lumbering 
up  and  down  course.  Punnett  thinks  these  differ- 
ences are  so  distinct  that  they  are  "  unlikely  to  be 
confounded  by  an  enemy  with  any  appreciation  of 
color  or  form." 

Moreover,  in  Ceylon  at  least,  the  distribution  of  the 
model  and  its  mimic  is  very  different  from  what  is 
expected  on  the  theory  of  mimicry.  He  concludes  that 
the  facts  relative  to  their  distribution  "are  far  from 
lending  support  to  the  view  that  the  polymorphic 
females  of  P.  polytes  owe  their  origin  to  natural  selec- 


128  HEREDITY  AND   SEX 

tion,  in  the  way  that  the  upholders  of  the  theory  of 
mimicry  would  lead  us  to  suppose." 

After  considering  the  difficulties  that  the  theory  of 
mimicry  has  to  contend  with,  Punnett  points  out  that 
dimorphic  and  polymorphic  species  are  not  uncommon 
in  butterflies,  and  that  in  many  of  these  cases  there  can 
be  little  or  no  question  of  mimicry  having  anything 


FIG.  65.—  Papilio  turnus  ;  female  (above)  and  male  (below),  and  the  variety 
P.  turnus  glaucus  (above,  right)  which  appears  only  in  the  female. 

to  do  with  the  matter.  It  is  well  known  that  in  Lepidop- 
tera  the  modified  form  commonly  belongs  to  the  female 
sex.  In  one  case  (Abraxas  grossulariata)  it  is  known 
that  the  aberrant  female  type  appears  sporadically,  as  a 
sport,  and  follows  Mendel's  law  of  segregation.  Punnett 
shows  how  the  recurrence  of  the  single  type  of  male  and 
the  three  types  of  females  of  polytes  may  also  be  ac- 
counted for  by  the  recognised  methods  of  Mendelian  in- 
heritance. He  points  out  that  by  the  assumption  that 


SECONDARY  SEXUAL  CHARACTERS  129 

these  types  have  suddenly  appeared  as  mutants  many 
of  the  difficulties  of  the  older  theories  are  avoided, 
and  that  such  an  assumption  is  in  harmony  with 
an  ever  increasing  body  of  evidence  concerning 
variation  and  heredity.  On  this  view  "  natural  se- 
lection" plays  no  part  in  the  formation  of  these 
polymorphic  forms,"  nor  does  sexual  selection.  The 
absence  of  transitional  forms  is  explicable  on  this 


FIG.  66.  —  Colias  philodice,  showing  two  female  forms  above  and 
one  male  form  below. 

view,  and  unaccountable  on  the  other  theory.  In 
fact  polymorphic  forms,  if  they  appear,  would  be 
expected  to  persist  if  they  are  not  harmful  to  the 
species. 

We  have  in  this  country  several  species  of  butter- 
flies in  which  polymorphism  exists.  In  the  north 
the  species  Papilio  turnus  (Fig.  65)  is  alike  in  the  male 
and  in  the  female.  But  in  the  south  two  types  of 
females  exist  —  one  like  the  male  and  the  other  a 
black  type. 


130  HEREDITY  AND   SEX 

In  the  Eastern  States  there  is  a  butterfly,  Colias 
philodice,  in  which  two  types  of  female  exist  (Fig.  66). 
Gerould  has  studied  the  mode  of  inheritance  of  these 
two  types  and  finds  that  they  conform  to  a  scheme  in 
which  the  two  females  differ  by  a  single  factor.  The  evi- 
dence is  strongly  in  favor  of  the  view  that  one  of  these 
forms  has  arisen  as  a  mutation.  There  is  no  need  to 
suppose  that  sexual  selection  has  had  anything  to  do 
with  its  origin,  and  no  evidence  that  it  owes  its  exist- 
ence* to  mimicry  of  any  other  species. 

Finally,  I  should  like  to  speak  of  a  case  that  has  come 
under  my  own  observation.  One  of  the  mutants  that 
appeared  in  a  culture  of  drosophila  had  a  new  eye 
color  that  was  called  eosin.  In  the  female  the  eye  is 
much  deeper  in  color  than  in  the  male.  The  race  main- 
tains itself  as  a  bicolor  type  without  any  selection. 

CONCLUSIONS 

In  conclusion  let  me  try  to  bring  together  the  main 
considerations  that  seem  to  me  to  throw  serious  doubts 
on  Darwin's  theory  of  sexual  selection. 

First.  Its  fundamental  assumption  that  the  evolution 
of  these  characters  has  come  about  through  the  "will," 
"choice,"  or  selection  of  the  female  is  questionable, 
because  of  want  of  evidence  to  show  that  the  females 
make  their  choice  of  mates  on  this  basis.  There  is  also 
some  positive  evidence  to  show  that  other  conditions 
than  selection  of  the  more  ornamented  individual 
(because  he  is  the  more  ornamental)  are  responsible 
for  the  mating. 

Second.  We  have  come  to  have  a  different  concep- 
tion of  what  selection  can  do  than  the  sliding  scale 


SECONDARY  SEXUAL   CHARACTERS  131 

assumption  that  has  been  current,  at  least  by  implica- 
tion,  in  much  of  the  post-Darwinian  writings. 

Third.  Recent  advances  in  the  study  of  variations 
have  given  us  a  new  point  of  view  concerning  the  na- 
ture of  variation  and  the  origin  of  variations.  If  we 
are  justified  in  applying  this  new  view  to  secondary 
sexual  characters,  the  problem  appears  greatly  sim- 
plified. 


CHAPTER  V 

THE  EFFECTS  OF   CASTRATION   AND    OF   TRANSPLAN- 
TATION ON  THE  SECONDARY  SEXUAL  CHARACTERS 

IN  several  of  the  preceding  chapters  I  have  spoken  in 
some  detail  of  sex-linked  inheritance.  In  sex-linked 
inheritance  we  deal  with  a  class  of  characters  that  are 
transmitted  to  one  sex  alone  in  certain  combinations, 
and  have  for  this  reason  often  been  called  sex-limited 
characters ;  but  these  same  characters  can  be  trans- 
ferred by  other  combinations,  as  we  have  seen,  to  the 
other  sex,  and  are  therefore  not  sex-limited. 

In  contrast  to  these  characters  Secondary  sexual  char- 
acters appear  in  one  sex  only  and  are  not  transferable 
to  the  other  sex  without  an  operation.  For  instance, 
the  horns  of  the  stag  and  the  colors  and  structures  of 
certain  male  birds  are  in  nature  associated  with  one 
sex  alone. 

It  has  long  been  recognized  in  mammals  and  birds 
that  there  is  a  close  connection  between  sexual  maturity 
and  the  full  development  of  the  secondary  sexual  char- 
acters. This  relation  suggests  some  intimate  correla- 
tion between  the  two.  It  has  been  shown,  in  fact,  in 
some  mammals  at  least,  that  the  development  of  the 
secondary  sexual  characters  does  not  take  place,  or 
that  they  develop  imperfectly,  if  the  sex  glands  are 
removed.  It  may  appear,  therefore,  that  we  are  deal- 
ing here  with  a  purely  physiological  process,  and  that 

132 


THE  EFFECTS   OF  CASTRATION  133 

the  development  of  these  structures  and  colors  is  a  by- 
product of  sex  itself,  and  calls  for  no  further  explana- 
tion. 

But  the  question  cannot  be  so  hastily  dismissed. 
This  can  best  be  shown  by  taking  up  at  once  the  ma- 
terial at  hand. 

OPERATIONS   ON   MAMMALS 

In  the  deer,  the  facts  are  very  simple.  If  the  very 
young  male  is  castrated  before  the  knobs  of  the  antlers 
have  appeared,  the  antlers  never  develop. 


FIG.  67.  —  Merino;  male  (horned)  and  female  (hornless). 

If  the  operation  is  performed  at  the  time  when  the 
antlers  have  already  begun  to  develop,  incomplete 
development  takes  place.  The  antlers  remain  covered 
by  the  velvet  and  are  never  thrown  off.  They  are  called 
peruke  antlers.  If  the  adult  stag  is  castrated  when 
the  horns  are  fully  developed,  they  are  precociously 


134  HEREDITY  AND   SEX 

dropped,  and  are  replaced,  if  at  all,  by  imperfect  ant- 
lers, and  these  are  never  renewed. 

These  facts  make  it  clear  that  there  is  an  intimate 
relation  between  the  orderly  sequence  of  development 
of  the  horns  in  the  deer  and  the  presence  of  the  male 
sexual  glands. 

In  the  case  of  sheep,  the  evidence  is  more  explicit. 
Here  we  have  carefully  planned  experiments  in  which 
both  sexes  have  been  studied ;  and  there  are  breeding 


FIG.  68.  —  Dorset;  male  (horned)  and  female  (horned). 

experiments  also,  in  which  the  heredity  of  horns  has 
been  examined. 

In  some  breeds  of  sheep,  as  in  the  Merinos  and 
Herdwicks,  horns  are  present  in  the  males,  absent  in 
the  females  (Fig.  67).  In  other  breeds  of  sheep,  as 
in  Dorsets,  both  males  and  females  have  horns  (Fig. 
68).  In  still  other  breeds  both  sexes  lack  horns,  as 
in  some  of  the  fat-tailed  sheep  of  Africa  and  Asia 
(Fig.  69). 

Marshall  has  made  experiments  with  Herdwicks  - 
a  race  of  sheep  in  which  the  rams  have  large,  coiled 
horns  and  the  ewes  are  hornless.  Three  young  rams 
(3,  4,  and  5  months  old)  were  castrated.  The  horns 
had  begun  to  grow  (3,  4^,  and  6  inches  long)  at  the 
time  of  operating.  They  ceased  to  grow  after  the 
operation. 


THE  EFFECTS   OF   CASTRATION  135 

A  similar  operation  was  also  carried  out  on  females. 
Three  Herdwick  ewe  lambs  (about  3  months  old)  were 
operated  upon.  After  ovariotomy,  the  animals  were 
kept  for  17  months,  but  no  horns  appeared,  although 
in  one,  small  scurs  developed,  in  the  other  two  scarcely 
even  these.  It  is  clear  that  the  removal  of  the  ovaries  j 
does  not  lead  to  the  development  of  horns  like  those 
in  the  male. 

Now,  the  interpretation  of  this  case  can  be  made 
only  when  taken  in  connection  with  experiments  in 
heredity.  There  is  a  crucial  experiment  that  bears  on 
this  question.  Arkell  found  when  a  IVJoritiu  ewj£-(a,  race 
with  horned  males  and  hornless  females)  was  bred  to  a 
ram  of  a  hornless  breed,  that  the  sons  had  horns.  In 
this  case  the  factor  for  horns  must  have  come  from  the 
hornless  mother,  while  the  development  of  the  horns  was 
made  possible  by  the  presence  of  the  male  glands,  'lib  ' 
is  evidentJiherefore  in  the-iastration  experiment  that 
a  factor  for  horns  is  inherited  by  both  sexes,  but  in  order 
that  the  horns  may  develop  fully,  the  male  glands  must 
be  present  and  functional.^ 

In  the  Dorset,  both  sexes  are  horned,  the  horns  of  the 
females  are  lighter  and  smaller  than  the  horns  of  the 
ram  (Fig.  68).  In  the  castrated  males  the  horns  are 
like  those  of  the  females.  In  this  case  we  must  sup- 
pose that  the  hereditary  factor  for  horns  suffices  to 
carry  them  to  the  point  in  development  reached  by  the 
females.  To  carry  them  further  the  presence  of  the  sex 
glands  of  the  male  is  necessary. 

In  the  case  of  the  hornless  breeds  I  do  not  know  of 
any  evidence  from  castration  or  ovariotomy.  We  may 
suppose,  either  that  the  factor  for  horns  is  absent ;  or, 


136  HEREDITY  AND   SEX 

if  present,  some  inhibitory  factor  must  bring  about  sup- 
pression of  the  horns.  The  former  assumption  seems 
more  probable,  for,  as  I  shall  point  out,  certain  experi- 
ments in  heredity  indicate  that  no  inhibitor  is  present 
in  hornless  breeds. 

The    series    is    completed    by    cases    like    that    of 
the  eland  and  the  reindeer.     Both  males  and  females 


FIG.  69.  —  Fat-tailed  hornless  sheep  (Ovis 
aries  steatopyga  persicci). 

have  well-developed  horns.  In  this  case  the  hereditary 
factors  suffice  in  themselves  for  the  complete  develop- 
ment of  horns,  for  even  after  castration  the  horns  de- 
velop. 

We  have  anticipated  to  some  extent  the  conclusions 
arrived  at  by  breeding  experiments  in  these  races  of 
sheep.  The  best-known  case  is  that  of  Wood,  who 
crossed  horned  Dorsets  and  hornless  Suffolks.  As 


THE   EFFECTS   OF  CASTRATION 


137 


shown  in  the  picture  (Fig.  70)  the  sons  had  horns  - 
the  daughters  lacked  them.     When  these  are  inbred, 
their  offspring  are  of  four  kinds,  horned  males,  hornless 
males,  horned  females,  hornless  females. 

It  seems  probable  that  these  four  classes  appear  in 
the  following  proportions : 


Horned 
3 


Hornless  S 
I 


Horned 
1 


Hornless 
3 


The  explanation  that  Bateson  and  Punnett  offer  for 
this  case  is  as  follows  :  The  germ-cells  of  the  horned  race 


FIG.  70.  —  1,  Suffolk  (ram),  hornless  in  both  sexes;  2,  Dorset  (ewe), 
horned  in  both  sexes ;  3,  FI  ram,  horned  ;  4,  Fi  ewe,  hornless ;  5-8,  the  four 
types  of  F* ;  5  and  6  are  rams,  7  and  8  are  ewes.  The  hornless  rams  are 
pure  for  absence  of  horns,  and  the  horned  ewes  are  pure  for  the  presence  of 
horns.  Figs.  5  and  6  represent  lambs.  (Bateson,  after  Wood.) 

(both  male  and  female)  carry  the  factor  for  horns  (H) ; 
the  germ-cells  of  the  hornless  race  lack  the  factor  for 
horns  (h).  The  female  is  assumed  to  be  homozygous 
for  the  sex  factor,  i.e.  two  sex  chromosomes  (X)  are 
present ;  while  the  male  has  only  one  sex  chromosome 


138  HEREDITY  AND   SEX 

carried  by  the  female-producing  sperm.  The  analysis 
is  then  as  follows:  One  "dose"  of  horns  (H)  in  the 
male  produces  horns,  but  two  doses  are  necessary  for 
the  female. 

Hornless  ?      hX  —  hX 

Horned  3       H  X  —  H 

Fi  HXhX    hornless  9 

H  h  X         horned    $ 


Gametes 
of  Fi 


Eggs    HX  —  hX 

Sperm  HX  —  hX  —  H  —  h 


-     F2  FEMALES  F2  MALES 

HXHX  horned  HEX  horned 

H  X  h  X   hornless  H  h  X  horned 

hXHX   hornless  HEX   horned 

h  X  h  X     hornless  h  h  X    hornless 

As  pointed  out  by  Punnett  a  test  of  the  correctness 
of  this  interpretation  is  found  by  breeding  the  FI 
hornless  female  to  a  hornless  male  (of  a  hornless  breed) . 
It  is  assumed  that  such  a  female  carries  the  factors  for 
horns  in  a  heterozygous  condition ;  if  so,  then  half  of 
her  sons  should  have  horns,  as  the  following  analysis 
shows : 

Fi  Hornless  ?     HX  —  hX 
Hornless  $     hX  —  h 

/*  X  #  X  hornless  T 

h  X  h  X   hornless  ? 

hEX      horned  <J 

hh  X       hornless  c? 


THE   EFFECTS  OF  CASTRATION 


139 


GP 


FIG.  71.  —  Upper  figure  normal  male  guinea  pig  (from  below),  to  show 
mammary  glands.  Lower  figure,  a  feminized  male ;  i.e.  castrated  when 
three  weeks  old  and  pieces  of  ovaries  transplanted  beneath  the  skin,  at  On. 


140  HEREDITY  AND   SEX 

The  actual  result  conforms  to  the  expectation.  The 
results  of  both  of  the  experiments  are  consistent  with 
the  view  that  one  factor  for  horns  in  the  male  produces 
horns,  which  we  may  attribute  to  the  combined  action 
of  the  inherited  factor  and  a  secretion  from  the  testes 
which  reenforces  the  action  of  the  latter.  This,  how- 
ever, should  be  tested  by  castrating  the  FI  males.  In 
the  females,  one  factor  for  horns  fails  to  produce  horns, 
while  two  factors  for  horns  cause  their  development. 

Aside  from  some  of  the  domesticated  animals  (horses, 
cattle,  dogs,  cats,  pigs),  the  only  other  mammals  on 
which  critical  experiments  have  been  made  —  if  we 
exclude  man  —  are  the  rat  and  the  guinea  pig.  The 
next  case  is  unique  in  that  tne  ovary  was  transplanted 
to  a  male. 

Stginach  removed  the  sex  glands  from  the  male 
guinea  pig  and  rat  and  transplanted  into  the  same 
animals  the  ovaries  of  the  female,  which  established 
themselves.  Their  presence  brought  about  remarkable 
effects  on  the  castrated  male.  The  mammary  glands, 
that  are  in  a  rudimentary  condition  in  the  male,  be- 
come greatly  enlarged  (Fig.  71).  In  the  rat  the  hair 
assumes  the  texture  of  that  of  the  female.  The  skele- 
ton is  also  more  like  that  of  the  female  than  the  male. 
The  size  of  the  feminized  rats  and  guinea  pigs  is  less 
than  that  of  normal  (or  of  castrated)  males  and 
like  that  of  the  female  (Fig.  72).  Finally,  in  their 
sexual  behavior,  the  feminized  rats  were  more  like 
females  than  like  males.  These  cases  are  important 
because  they  are  the  only  ones  in  which  success- 
ful transplanting  of  the  ovary  into  a  male  has  been 
accomplished  in  vertebrates. 


THE  EFFECTS  OF  CASTRATION 


141 


V 


FIG.  72.  —  Two  upper  figures,  normal  male  guinea  pig  to  left,  M,  and 
his  brother,  F,  to  right  —  a  feminized  male.  Two  middle  and  two  lower 
figures,  a  normal  male  at  J/,  and  his  feminized  brother,  F.  (After  Steinach.) 


142  HEREDITY  AND   SEX 

OPERATIONS   ON   BIRDS 

In  striking  contrast  to  these  results  with  mammals 
are  those  with  birds,  where  in  recent  years  we  have 
gained  some  definite  information  concerning  the  devel- 
opment of  secondary  sexual  characters. 

I  am  fortunate  in  being  able  to  refer  to  several 
cases  —  the  most  successful  on  record  —  carried  out 
by  my  friend,  H.  D.  Goodale,  at  the  Carnegie  Lab- 
oratory at  Cold  Spring  Harbor.  One  "case"  is  that 
of  a  female  Mallard  duck  from  which  the  ovary  was 
completely  removed  when  she  was  a  very  young  bird. 
Figure  16  illustrates  the  striking  difference  between 
the  normal  male  and  the  female  Mallard.  In  the 
spayed  female  the  plumage  is  like  that  of  the  male. 

Darwin  records  a  case  in  which  a  female  duck  in  her 
old  age  assumed  the  characteristics  of  a  male,  and 
similar  cases  are  recorded  for  pheasants  and  fowls. 

Goodale  also  removed  the  ovary  from  very  young 
chicks.  He  found  that  the  female  developed  the 
secondary  sexual  plumage  of  the  cock. 

How  shall  we  interpret  these  cases  ?  It  is  clear  that 
the  female  has  the  potentiality  of  producing  the  full 
plumage  of  the  male,  but  she  does  not  do  so  as  long  as 
the  ovary  is  present.  The  ovary  must  therefore  be 
supposed  to  prevent,  or  inhibit,  the  development  of 
secondary  sexual  characters  that  appear  therefore  only 
in  the  male. 

The  converse  operation  —  the  removal  of  the  male 
glands  from  the  male  —  is  an  operation  that  is  very 
common  among  poultrymen.  The  birds  grow  larger 
and  fatter.  They  are  known  as  capons.  In  this  case 


THE   EFFECTS   OF   O^TRATION 


143 


/ 

the  male  assumes  his  full  nprm^l  plumage  with  all  of 

his  secondary  male  sexual^  characters.  It  is  said  that 
the  comb  and  wattles  and  to  soi/ie  extent  the  spurs  are 
less  developed  in  the  capon  thadi  in  the  normal,  male. 
But  aside  from  this  it  is /quite  certain  that  the  de- 
velopment of  the  secondary  sexual  plumage  in  the 

// 


FIG.  73.  —  Male  and  female  Seabright.  Note  short  neck  feathers  and 
incomplete  tail  cover  in  male.  In  the  Seabright  cock  the  sickle  feathers  on 
back  at  base  of  tail  are  like  those  of  the  hen.  (After  "  Reliable  Poultry 
Journal.") 

male  is  largely  independent  of  the  presence  of  the  sex 
glands.    ^, 

The  method  of  inheritance  of  the  secondary  sexual 
characters  in  birds  has  been  little  studied.  Daven- 
port has  reported  one  case,  but  I  am  not  sure  of  his  in- 
terpretation.1 I  have  begun  to  study  the  question  by 
using  Seabright  bantams,  in  which  the  male  lacks  some 

1  Because  it  is  not  evident  whether  the  secondary  sexual  char- 
acters as  such  are  involved  or  only  certain  general  features  of 
coloration. 


144  HEREDITY  AND   SEX 

of  the  secondary  sexual  characters  of  the  domestic 
races,  notably  the  saddle  feathers,  as  shown  in  Fig.  73. 
A  male  Seabright  was  mated  to  a  black-breasted  game 
female.  The  son  was  hen-feathered  and  like  the  Sea- 
bright  father  in  this  respect.  Evidently  in  this  case 
the  secondary  sexual  character  in  question  is  dominant 
and  is  transmitted  from  father  to  son. 

In  the  reciprocal  cross  one  hen  was  obtained  which 
was  back-crossed  to  a  recessive  male.  She  produced 
both  hen-feathered  and  normally  feathered  sons.  The 
character  appears  therefore  to  be  sex-limited  but  not 
sex-linked.  If  hen-feathering  in  the  Seabright  be  rep- 
resented by  S  and  its  normal  allelomorph  by  s,  the 
first  cross  would  be  as  follows :  — 

Game  ?  sF — s 

Seabright  <J         S  —  S 

SsF      female 

Ss         hen-feathered  male 

Eggs  of  Fl  SF  —  sF  —  S  —  s 

Sperm  of  FI  S  —  s 

F2  Females        F2  Males 
SSF  SS    hen-feathered 

SsF  Ss     hen-feathered 

sSF  sS     hen-feathered 

ss       cock-feathered 


In  conclusion,  then,  in  mammals  the  secondary  sexual 
characters  owe  their  development  to  the  testes.  -  The 
testes  add  something  to  the  common  inheritance. 
But  in  birds  the  ovary  takes  something  away. 


THE  EFFECTS  OF  CASTRATION  145 

OPERATIONS   ON   AMPHIBIA 

The  male~triton  develops  each  year  a  peculiar  fin  or 
comb  on  the  back  and  tail.  Bresca  has  found  that 
after  castration  the  comb  does  not  develop.  If  present 
at  the  time  of  castration,  the  comb  is  arrested,  but 
only  after  several  months.  Certain  color  marks  pe- 
culiar to  the  male  are  not  lost  after  castration.  If  the 
comb  is  removed  in  normal  males,  it  regenerates,  but 
less  perfectly  in  castrated  males.  If  a  piece  of  the 
dorsal  fin  of  the  female  is  transplanted  to  a  normal  male 
in  normal  position,  it  may  later-  produce  the  comb  under 
the  influence  of  the  testes. 

In  the  frog  there  appears  at  the  breeding  season  a 
thickening  of  the  thumb.  Castrated  males  do  not 
produce  this  thickening. 

If  it  is  present  in  a  male  at  the  time  of  castration  it 
is  thrown  off,  according  to  Nussbaum,  but  according  to 
Smith  and  Shuster  its  further  progress  only  is  arrested. 
According  to  Nussbaum  and  Meisenheimer  injection 
of  pieces  of  testes  beneath  the  skin  of  a  castrated  male 
cause  the  thumb  development  to  take  place,  or  to 
continue,  but  Smith  and  Shuster  question  this  con- 
clusion. 

Such  are  the  remarkable  relations  that  these  experi- 
ments have  brought  to  light.  How,  we  may  ask,  do 
the  sex  glands  produce  their  effect,  in  the  one  case  to 
add  something,  in  the  other  to  suppress  something? 

It  has  often  been  suggested  these  glands  produce 
their  effects  through  the  nervous  system  by  means  of 
the  nerves  to  or  from  the  reproductive  organs.  This 
has  been  disproved  in  several  cases  by  cutting  the 


146  HEREDITY  AND  SEX 

nerves  and  isolating  the  glands.     The  results  are  the 
same  as  when  they  are  left  intact. 

This  brings  us  to  one  of  the  most  interesting  chapters 
of  modern  physiology,  the  production  and  influence  of 
Internal  Secretions. 

INTERNAL    SECRETIONS 

It  has  become  more  and  more  probable  that  the  effects 
in  question  are  largely  brought  about  by  internal  se- 
cretions of  the  reproductive  organs.  These  secretions 
are  now  called  " hormones"  or  " exciters."  They  are 
produced  not  only  by  glands  that  have  ducts  or  outlets, 
but  by  many,  perhaps  by  all,  organs  of  the  body.  Some 
of  these  secretions  have  been  shown  to  have  very  re- 
markable effects.  A  few  instances  may  be  mentioned 
by  way  of  example. 

The  pituitary  body  produces  a  substance  that  has  an 
important  influence  on  growth.  If  the  pituitary  body 
becomes  destroyed  in  man,  a  condition  called  gigan- 
tism  appears.  The  bones,  especially  of  the  hands  and 
\  feet  and  jaws,  become  enlarged.  The  disease  runs  a 
short  course,  and  leads  finally  to  a  fatal  issue. 

The  thyroid  and  parathyroid  bodies  play  an  im- 
portant role  in  the  economy  of  the  human  body 
through  their  internal  secretions.  Removal  leads  to 
death.  A  diseased  condition  of  the  glands  is  asso- 
ciated with  at  least  six  serious  diseases,  amongst  them 
cretinism. 

The  thymus  secretion  is  in  some  way  connected  with 
the  reproductive  organs.  Vincent  suggests  that  "the 
thymus  ministers  to  certain  needs  of  the  body  before 
the  reproductive  organs  are  fully  developed." 


THE  EFFECTS  OF  CASTRATION  147 

Extirpation  of  the  adrenal  bodies,  another  ductless 
gland,  leads  to  death.  Injury  to  these  bodies  causes 
Addison's  disease. 

Finally,  the  reproductive  glands  themselves  produce 
internal  secretions.  In  the  case  of  the  male  mammal  it 
has  been  shown  with  great  probability  that  it  is  the 
supporting  tissues  of  the  glands,  and  not  the  germ-cells, 
that  produce  the  secretion.  Likewise,  in  the  case  of 
the  ovary,  it  appears  that  the  follicle  cells  of  the  corpus 
luteum  give  rise  to  an  important  internal  secretion. 
If  the  sac-like  glands  are  removed,  the  embryo  fails  to 
become  attached  to  the  wall  of  the  uterus  of  the  mother. 
If  the  ovary  itself  is  removed  from  a  young  animal, 
before  corpora  lutea  are  formed,  the  uterus  remains  in 
an  infantile  condition. 

From  a  zoological  point  of  view  the  recent  experi- 
ments of  Gudernatsch  are  important.  He  fed  young 
frog  tadpoles  with  fresh  thyroid  glands.  "  They  began 
very  soon  to  change  into  frogs,  but j^aseiLto^grow  in 
size.  The  tadpoles  might  begin  tEeir  metamorphosis 
in  a  few  days  after  the  first  application  of  the  thyroid, 
and  weeks  before  the  control  animals  did  so." 

In  contrast  to  these  effects  Gudernatsch  found  that 
tadpoles  fed  on  thymus  grew  rapidly  and  postponed 
metamorphosis.  They  might  even,  in  fact,  fail  to 
change  into  frogs  and  remain  permanently  in  the  tad- 
pole condition.  If  thyroid  extracts  produce  dwarfs; 
thymus  extracts  make  giant  tadpoles  that  never  become 
adults. 

These  examples  will  suffice  to  show  some  of  the  im- 
portant effects  on  growth  that  these  internal  secretions 
may  bring  about. 


148  HEREDITY  AND   SEX 

OPERATIONS    ON    INSECTS 

The  Insects  constitute  the  third  great  group  in  which 
secondary  sexual  characters  are  common. 

The  first  operations  on  the  reproductive  organs  were 
carried  out  by  Oudemans  on  the  gipsy  moth,  Ocneria 
(Porthetria)  dispar.  The  male  anoTlemale  are  strik- 
ingly different.  Oudemans  removed  the  testes  from 


FIG.  74.  —  Ovaries  of  Lymantria  (Porthetria}  dispar  transplanted  to  male. 
They  have  established  connection  with  the  sperm  ducts.     (After  Kopec.) 

young  caterpillars  and  found  no  change  in  the  color,  or 
size,  of  the  male.  He  also  removed  the  ovaries  from 
young  caterpillars,  and  again  found  no  effect  in  the  fe- 
male. The  same  experiments  were  later  carried  out  on 
a  large  scale  by  Meisenheimer,  who  obtained  similar 
results.  Meisenheimer  went  further,  however,  and  per- 
formed another  operation  of  great  interest.  He  removed 
the  male  glands  from  a  male  and  implanted  in  their 


THE   EFFECTS  OF  CASTRATION 


149 


place  the  ovary  of  a  female,  while  it  was  still  in  a  very 
immature  condition.  The  caterpillar  underwent  its 
usual  growth,  changed  to  a  chrysalid,  and  then  to  a 
moth.  The  moth  showed  the  characters  of  the  .male. 
The  presence  of  the  ovary  had  produced  no  effect  what- 
ever on  the  body  character  of  the  individual.  When 
this  individual  was  dissected,  Meisenheimer  found  that 
the  ovary  had  completely  developed.  It  contained 
mature  eggs,  and  the  ovary  had  often  established  con- 
nection with  the  outlets  of  the  male  organs  that  had 


FIG.  75.  —  Testes  of  Lymantria  (Porthetria)  dispar  transplanted  to  female. 
They  have  connected  with  the  oviducts.     (After  Kopec.) 

been  left  behind,  as  seen  in  Fig.  74,  taken  from  Kopec's 
description. 

The  converse  experiment  was  also  made.  The  ovaries 
were  removed  from  young  caterpillars,  and  in  their 
place  were  implanted  the  male  sex  glands  from  a  young 
male  caterpillar.  Again  no  effects  were  produced  on  the 
moth,  which  showed  the  characteristic  female  size  and 
color.  On  dissection  the  testes  were  also  found  to  have 
grown  to  full  size  and  to  have  produced  spermatozoa 
(Fig.  75). 

These  remarkable  results,  confirmed  by  Kopec,  show 


150 


HEREDITY  AND   SEX 


that  in  these  insects  the  essential  organs  of  reproduc- 
tion have  no  influence  on  the  secondary  sexual  char- 
acters of  the  individual.  They  show  furthermore  that 
the  male  generative  organs  will  develop  as  well  in 
the  female  as  in  the  body  of  the  male  itself,  and  vice 
versa. 

It  is  evident,  then,  in  insects  (there  is  a  similar,  but 
less  complete,  series  of  experiments  on  the  cricket), 


FIG.  76.  — Papilio  Memnon.     1,  male;    2,  3,  4,  three  types  of  females. 
(After  Meijere.) 

that  the  heredity  of  the  secondary  sexual  characters 
can  be  studied  quite  apart  from  the  influence  of  the 
sex  glands.  How,  then,  are  they  inherited  so  that  they 
appear  in  one  sex  and  not  in  the  other  sex?  Within 
the  last  two  or  three  years  the  inheritance  of  the  second- 
ary sexual  differences  in  insects  has  been  studied. 

First,  there  is  the  case  of  the  clover  butterfly,  Colias 
philodice,  that  Gerould  has  worked  out,  where  there 


THE  EFFECTS  OF  CASTRATION  151 

are  two  types  of  females  and  one  kind  of  male 
(Fig.  66).  * 

Without  giving  the  analysis  of  this  case  I  may  say 
that  the  results  can  be  explained  on  a  Mendelian  basis. 
The  peculiar  feature  of  Gerould's  explanation  is  that 
two  doses  of  the  yellow-producing  determiner  in  the 
female  give  yellow  color  —  one  dose  gives  white.  In 
the  male,  on  the  other  hand,  one  dose  of  yellow  gives 
yellow. 

The  second  case  is  that  of  PjipiJ^memnon,  worked  out 
by  de  Meijere  from  the  experiments  of  Jacobson.  There 
is  one  male  type  and  three  female  types,  Fig.  76.  De 
Meijere  accounts  for  the  results  of  matings  in  this 
species  recorded  by  Jacobson  on  the  assumption  of 
three  factors,  one  for  each  type  of  female.  The  three 
factors  are  treated  as  allelomorphs,  and  therefore  only 
two  of  them  can  be  present  in  any  one  individual,  and 
since  they  are  allelomorphs  they  will  pass  into  different 
gametes.  The  order  of  dominance  is  Achates,  Agenor, 
Laomedon.  The  male  carries  these  same  factors,  but 
they  are  not  effective  in  him.  Baur  accounts  for  the 
results  in  a  somewhat  different  way,  but  involving  or- 
dinary Mendelian  conceptions. 

An  interesting  case  is  that  reported  by  Foot  and 
Strobell.  They  crossed  a  female  of  a  bug,  Euschistw 
variolarius,  the  male  of  which  has  a  black  spot  on  the 
end  of  the  body  (the  female  lacking  the  spot),  with  a 
male  of  Euschistus  servus  that  lacks  the  spot  both  in 
the  males  and  the  females  (Fig.  77).  The  daughters 
had  no  spot ;  the  sons  had  a  faint  spot,  less  developed 
than  in  variolarius.  When  these  (Fi)  offspring  were 
inbred,  they  obtained  249  females  without  a  spot, 


152  HEREDITY  AND   SEX 

107  males  with  a  spot  (developed  to  different  degrees), 
and  84  males  without  a  spot.  The  authors  give  no 
explanation  of  their  results  —  but  they  use  the  re- 


FIG.  77.  —  To  left,  in  1,  is  male  of  Euschistus  variolarius,  to  right  male  of 
E.  serous.  2  and  3  show  eight  F2  males;  4  shows  seven  F2  males  from 
another  mating.  (After  Foot  and  Strobell.) 

suits  to  discredit  some  of  the  explanations,  that  rest 
on  the  assumption  that  the  chromosomes  are  the  chief 
factors  in  Mendelian  heredity.  I  venture,  neverthe- 
less, to  suggest  the  explanation  shown  on  the  accom- 


THE  EFFECTS  OF  CASTRATION  153 

panying  diagram  (Fig.  78).  The  analysis  rests  on  the 
assumption  that  neither  one,  nor  two  doses  of  S  in  the 
female  is  able  to  produce  a  spot,  while  in  the  male  one 
dose  of  S  suffices. 

E.  variolarius  ?        SX  --SX 
E.  servus  $  sX  —  s 


F, 

.sXSX    spotless  $ 
sXS       spotted  $ 

Gametes 
ofF, 

(Eggs    sX  —  SX 
(Sperm  sX  —  SX  —  s  —  S 

sXsX 

sXSX 
SXsX 

SXSX  , 

spotless  ? 

sXs  spotless  c? 

sXS  spotted  g 

SXs  spotted  (J 

SXS  spotted  (J 

FIG.  78.  —  Diagram  to  show  inheritance  of  spot  when  E.  rariplarius  (?) 
is  mated  to  E.  servus  (cT).  S  =  spot.  s=  no  spot.  X  =  sex  chromosome, 
that  does  not  carry  the  factor  S  for  spot. 

It  is  very  important  to  understand  just  what  is  meant 
by  this ;  for  otherwise  it  may  seem  only  like  a  restate- 
ment of  the  facts.  In  the  F2  female  with  the  formulae 
SXSX,  with  two  doses  of  the  S  factor,  no  spot  is  as- 
sumed to  appear  (nor  in  the  hybrid  female  SXsX).  At 
first  sight  it  seems  that  a  female  having  the  formula 
SXSX  is  only  double  the  male  with  sXSy  especially  if 
small  s  is  interpreted  to  mean  absence  of  spots.  But 
this  view,  in  fact,  involves  a  misconception  of  what  the 
factorial  hypothesis  is  intended  to  mean. 


154  HEREDITY  AND   SEX 

To  make  this  clearer,  I  have  written  out  the  case 
more  fully : 

X  ABC S        X  ABCS  ? 
X  ABC  S  ABC  s  S 

In  this,  as  in  all  such  Mendelian  formulae,  the  result 
(or  character)  that  a  factor  produces  depends  on  its 
relations  to  other  things  in  the  cell  (here  ABC).  We 
are  dealing,  then,  not  with  the  relation  of  X  to  S  alone, 
but  this  relation  in  turn  depends  on  the  proportion  of 
both  X  and  S  to  A  B  C.  It  is  clear,  if  this  is  admitted, 
that  the  two  formulae  above  —  the  one  for  the  male 
and  the  other  for  the  female  —  are  neither  identical 
nor  multiples. 

It  will  be  noted  that  in  only  one  of  these  attempts  to 
explain  in  insects  the  heredity  of  the  secondary  sexual 
characters  have  the  factors  for  the  characters  been 
assumed  to  be  carried  by  the  sex  chromosomes.  If 
one  accepts  the  chromosome  basis  for  heredity,  these 
results  may  be  explained  on  the  assumption  that  the 
factors  lie  in  other  chromosomes  than  the  sex  chromo- 
somes. 

In  the  next  case,  however,  that  I  shall  bring  forward 
the  factors  must  be  assumed  to  be  in  the  sex 
chromosomes  themselves. 

The  mutant  of  drosophila  with  eosin  eyes  that  arose 
in  my  cultures  is  the  case  in  question.  The  female 
has  darker  eyes  than  the  male.  The  experimental 
evidence  shows  that  the  factor  for  eosin  is  carried 
by  the  sex  chromosomes.  In  the  female  it  is  present, 
therefore,  in  duplex,  or,  as  we  say,  in  two  doses;  in 
the  male  in  one  dose. 


THE  EFFECTS  OF  CASTRATION  155 

The  difference  in  color  can  be  shown,  in  fact,  to  be 
due  to  this  quantitative  relation.  If,  for  instance,  an 
eosin  female  is  mated  to  a  white-eyed  male,  her 
daughters  have  light  eyes  exactly  like  those  of  the 
eosin  male.  The  white-eyed  fly  lacks  the  eosin  factor 
in  his  sex  chromosomes  (as  suitable  matings  show), 
hence  the  hybrid  female  has  but  one  dose  of  eosin, 
and  in  consequence  her  eye  color  becomes  the  same  as 
the  male. 

In  this  case  a  sex-linked  character  is  also  a  secondary 
sexual  character  because  it  is  one  of  the  rather  unusual 
cases  in  which  a  factor  in  two  doses  gives  a  stronger 
color  than  it  does  in  one  dose. 

PARASITIC    CASTRATION    OF    CRUSTACEA 

Let  us  turn  now  to  a  group  in  which  nature  performs 
an  interesting  operation. 

Giard  first  discovered  that  when  certain  male  crabs 
are  parasitized   by  another   crustacean,   sacculina   (a . 
cirriped  or  barnacle) ,  they  develop  the  secondary  sexual 
characters  of  the  female.     Geoffrey  Smith  has  confirmed 
these  results  and  carried  them  further  in  certain  re- 
spects.     Smith   finds  that   the   spider   crab,   Inachus* 
mauritanicus,  is  frequently  infected  by  Sacculina  neglecta 
(Fig.  79).     The  parasite  attaches  itself  to  the  crab  and  , 
sends  root-like  outgrowths  into  its  future  host.     These 
roots  grow  like  a  tumor,  and  send  ramifications  to  all 
parts  of  the  body  of  the  crab. 

The  chief  effect  of  the  parasite  is  to  cause  complete 
or  partial  atrophy  of  the  reproductive  organs  of  the 
crab,  and  also  to  change  the  secondary  sexual  charac- 
ters. Smith  says  that  of  1000  crabs  infected  by 


156 


HEREDITY  AND   SEX 


sacculina,   70%   of   both    males    and  females  showed 
alterations  in  their  secondary  sexual  characters. 

As  a  control,  5000  individuals  not  infected  were  ex- 
amined, only  one  was  unusual,  and  this  one  was  a  her- 
maphrodite (or  else  a  crab  recovered  from  its  parasite). 


FIG.  79.  - — A  male  of  Inachus  mauritanicus  (upper  left  hand).  Female  of 
Inachus  scorpi  (lower  left  hand) .  Male  of  Inachus  mauritanicus  carrying  on 
its  abdomen  two  specimens  of  Danalia  curvata  and  a  small  Sacculina  neglecta 
(upper  right  hand).  Male  of  Inachus  mauritanicus  with  a  Sacculina  neglecta 
on  it  (lower  right  hand).  The  abdomen  and  chelae  of  the  host  are  inter- 
mediate in  character  between  those  of  an  ordinary  male  and  female.  (After 
Geoffrey  Smith.) 


As  the  figures  (Fig.  80)  show,  the  adult  male  has 
large  claws ;  the  female,  small  ones.  He  has  a  narrow 
abdomen;  she  has  a  broad  one.  In  the  male  there 
is  a  pair  of  stylets  on  the  first  abdominal  ring  (and 
a  pair  of  greatly  reduced  appendages  behind  them). 
The  adult  female  has  four  biramous  abdominal  append- 
ages with  hairs  to  carry  the  eggs. 


THE  EFFECTS   OF  CASTRATION 


157 


^  & 


FIG.  80,  —  1,  adult  normal  male;  2,  under  side  of  abdomen  of  normal 


158  HEREDITY  AND   SEX 

The  infected  males  "show  every  degree  of  modi- 
fication towards  the  female  type."  The  legs  are  small, 
the  abdomen  broad,  the  stylets  reduced,  and  the 
typical  biramous  appendages  with  hairs  appear. 

When  the  female  crab  is  infected  she  does  not  change 
" toward"  the  male  type,  although  the  ovary  may 
be  destroyed.  The  only  external  change  is  that  the 
abdominal  appendage  may  be  reduced. 

In  a  hermit  crab,  Eupagurus  meticulosus,  infected  by 
Peltogaster  curvatus,  similar  results  have  been  obtained. 
The  infected  male  assumed  the  ordinary  sexual  char- 
acters of  the  female,  but  the  females  showed  no  change 
towards  the  male. 

In  these  cases  it  seems  probable  that  the  testes  of 
the  male  suppress  the  development  of  the  secondary 
sexual  characters  that  appear  ordinarily  only  in  the 
females.  The  case  is  the  reverse  of  that  of  the  birds 
and  different  again  from  that  of  the  mammals. 

In  birds  and  mammals  the  secondary  sexual  charac- 
ters are  in  many  cases  directly  dependent  on  the  in- 
ternal secretions  of  the  sex  glands.  These  secretions  are 
carried  alike  to  all  parts  of  the  body,  hence  the  absence 
of  bilateral  gynandromorphs  in  these  groups. 

adult  male;  3,  male  infected  with  sacculina,  showing  reduction  of  chela 
and  slight  broadening  of  abdomen;  4,  5,  showing  attenuated  copulatory  styles 
and  slight  hollowing  out  of  abdomen;  6,  under  side  of  abdomen  of  a  similar 
male  specimen,  showing  reduction  of  copulatory  styles  and  presence  of 
asymmetrically  placed  swimmerets  characteristic  of  female;  7,  infected 
male  which  has  assumed  complete  female  appearance;  8,  under  side  of 
abdomen  of  7,  showing  reduced  copulatory  styles  and  swimmerets;  9,  under 
aide  of  abdomen  of  similar  male  specimen  with  well-developed  copulatory 
styles  and  swimmerets;  10,  adult  female,  normal;  11,  under  side  of  abdomen 
of  10,  showing  swimmerets  and  trough-shaped  abdomen;  12,  under  side  of 
abdomen  of  infected  female,  showing  reduction  of  swimmerets;  13,  immature 
female  showing  small  flat  abdomen;  14,  under  side  of  abdomen  of  13, 
showing  flat  surface  and  rod-like  swimmerets.  (After  Geoffrey  Smith.) 


THE  EFFECTS  OF  CASTRATION  159 


CONCLUSIONS 

In  conclusion  it  is  evident  that  the  secondary  sexual 
characters  in  four  great  groups,  viz.  mammals,  birds, 
Crustacea,  and  insects,  are  not  on  the  same  footing. 
Their  development  depends  on  a  different  relation  to 
the  reproductive  organs  in  three  of  the  groups,  and  is 
independent  of  the  reproductive  organs  hi  the  fourth. 
It  is  not  likely,  therefore,  that  their  evolution  can  be 
explained  by  any  one  theory,  even  by  one  so  broad  in 
its  scope  as  that  of  sexual  selection. 

If,  for  example,  in  the  mammals  a,  mnrp  vigorous 
male,  due  to  greater  development  of  the  testes,  were 
"  selected  "  by  a  female,  the  chances  are  that  his  second- 
ary sexual  characters  will  be  better  developed  than  are 
those  of  less  vigorous  males,  but  he  is  selected,  not  on  this 
account,  but  hppansp  of  his  vigor.  If  a  male  bird  were 
"selected"  on  account  of  greater  vigor,  it  does  not 
appear  that  his  secondary  sexual  characters  would 
be  more  excessively  developed  than  those  of  less  vig- 
orous males,  provided  that  his  vigor  were  due  to  the 
early  or  greater  development  of  the  testes.  If  in 
birds  the  male  by  selecting  the  female  has  brought 
about  the  suppression  of  the  male  plumage,  which  is 
their  common  inheritance,  he  must  have  done  so  by 
selecting  those  females  whose  ovaries  produced  the 
greatest  amount  of  internal  secretions  which  suppresses 
male-feathering.  Moreover,  he  must  have  selected, 
not  fluctuating  variations,  but  germinal  variations. 
"Tn  insects  the  development  of  the  secondary  sexual 
characters  is  not  connected  with  the  condition  of  the 
reproductive  organs,  but  is  determined  by  the  complex 


160  HEREDITY  AND  SEX 

of  factors  that  determines  sex  itself.  If  selection  acts 
here,  it  must  act  directly  on  germinal  variations,  that 
are  independent  in  origin  of  the  sex-determining  factor, 
but  dependent  on  it  for  their  expansion  or  suppression. 

These  considerations  make  many  of  the  earlier  state- 
ments appear  crude  and  unconvincing ;  for,  they  show 
that  the  origin  of  the  secondary  sexual  characters  is 
a  much  more  complex  affair  than  was  formerly  im- 
agined. 

These  same  considerations  do  not  show,  however, 
that  if  a  new  germinal  character  appeared  that  gave 
its  possessor  some  advantage  either  by  accelerating 
the  opposite  sex  to  quicker  mating  or  by  being  corre- 
lated with  greater  vigor  and  thereby  making  more 
certain  the  discovery  of  a  mate,  such  a  character  would 
not  have  a  better  chance  of  perpetuation.  But  in 
such  a  case,  the  emphasis  no  longer  lies  on  the  idea 
of  selection  with  its  emotional  implications,  but  rather 
on  the  appearance  of  a  more  effective  machine  that 
has  arisen,  not  because  of  selection,  but,  having  arisen 
quite  apart  from  any  selective  process,  has  found  itself 
more  efficient.  Selection  has  always  implied  the  idea 
that  it  creates  something.  Now  that  the  evidence 
indicates  that  selection  is  not  a  guaranteed  method 
of  creating  anything,  its  efficiency  as  a  means  of  easy 
explanation  is  seriously  impaired. 


CHAPTER  VI 

GYNANDROMORPHISM,  HERMAPHRODITISM, 
PARTHENOGENESIS,  AND  SEX 

THREE  different  sex  conditions  occur  in  animals  and 
plants  that  have  a  direct  bearing  on  problems  of 
Heredity  and  Sex. 

The  first  condition  is  called  GyjiandrorjaLO_rphism  - 
a  condition  in  which  one  part  of  the  body  is  like  the 
male,  and  the  other  part  like  the  female. 
i  v  The  second  condition  is  called  Hermaphroditism  - 
a  condition  in  which  the  individuals  of  a  species  are  all 
alike  —  maleness    and    femaleness    are    combined    in 
the  same  body.     Two  sets  of  reproductive  organs  are 
present. 

The    third    condition    is    called    Parthenogenesis  - 
a  condition  in  which  the  eggs  of  an  animal  or  plant 
develop  without  being  fertilized. 

GYNANDROMORPHISM 

Gynandromorphs  occur  most  frequently,  in  fact 
almost  exclusively,  in  insects,  where  more  than  one 
thousand  such  individuals  have  been  recorded.  They 
are  most  abundant  in  butterflies,  common  in  bees 
(Fig.  81)  and  ants,  rarer  in  other  groups.  They 
occur  relatively  more  often,  when  two  varieties,  or 
species,  are  crossed,  and  this  fact  in  itself  is  signifi- 
cant. A  few  examples  will  bring  the  cases  before  us. 

In  my  cultures  of  fruit  flies  several  gynandro- 

161 


162  HEREDITY  AND   SEX 

morphs  have  arisen,  of  which  two  examples  are  shown 
in  Fig.  82.  In  the  first  case  the  fly  is  female  on  one 
side,  as  shown  by  the  bands  of  her  abdomen,  and  male 
on  the  other  side  (upper  right-hand  drawing). 

In  the  second  case  the  fly  looked  like  a  female  seen 
from  above.  But  beneath,  at  the  posterior  end,  the 
genital  organs  of  the  male  are  present,  and  normal 


FIG.  81.  —  A  gynandromorph  mutillid  wasp,  Pseudomethoca  canadensis, 
male  on  right  side,  female  on  left  side.     (After  Wheeler.) 

in  structure.  In  the  latter  case  the  fly  is  ostensibly 
a  female,  except  for  the  male  organs  of  reproduction. 

How  can  we  interpret  these  cases?  We  find  a 
clue,  I  think,  in  the  bee.  It  is  known  that  if  the  egg 
of  the  bee  is  fertilized,  it  produces  a  female  —  only 
female-producing  sperms  are  formed.  If  it  is  un- 
fertilized, it  produces  a  male.  In  the  bee  two  polar 
bodies  are  produced,  and  after  their  extrusion  the  num- 
ber of  chromosomes  is  reduced  to  half,  as  in  ordinary 
cases.  The  haploid  number  produces  a  male;  the 
double  number  produces  a  female. 

Boveri  pointed  out  #iat'*P  through  any  chance  the 


GYNANDROMORPHISM 


163 


entering  sperm  should  fail  to  reach  the  egg  nucleus 
before  it  divides,  it  may  then  fuse  with  one  of  the 
halves  of  the  egg  nucleus  after  that  divides.  From  the 


V 


FIG.  82.  —  Two  gynandromorphs  of  Drosophila  ampelophila.  Upper 
left-hand  figure,  female  dorsally,  male  ventrally  (as  seen  in  third  figure, 
lower  line).  Upper  right-hand  figure,  male  on  left  side,  female  on  right, 
and  correspondingly  the  under  side  shows  the  same  difference  (lower  row, 
last  figure  to  right.  Lower  row  from  left  to  right;  normal  female,  normal 
male,  vertical  gynandromorph  and  lateral  gynandromorph. 

half  of  the  egg  containing  the  double  nuclei  female 
structures  will  develop ;  frojjri  the  other  half,  contain- 
ing the  half  number  of  chromosomes,  male  structures 
(Fig.  83,  A).  Here  we  ha^a  very  simple  explanation 
of  the  gynandromorphism. 


164 


HEREDITY  AND   SEX 


There  is  another  way  in  which  we  may  imagine  that 
the  results  are  brought  about.     It  is  known  that  two  or 


FIG.  83.  —  Diagram,  illustrating  on  left  (A)  Boveri's  hypothesis,  on  right 
(B)  the  author's  hypothesis,  of  gynandromorphism. 

more  spermatozoa  frequently  enter  the  egg  of  the  bee. 
Should  only  one  of  them  unite  with  the  egg  nucleus, 
the  parts  that  descend  from  this  union  will  be  female. 
If  any  of  the  outlying  sperm  should  also  develop, 


GYXAXDROMORPHISM  165 

they  may  be  supposed  to  produce  male  structures 
(Fig.  83,  B). 

The  first  case  of  the  fly,  in  which  one  half  the  body 
is  male  and  the  other  female,  would  seem  better  in 
accord  with  Boveri's  hypothesis.  In  its  support 
also  may  be  urged  the  fact  that  Boveri  and  Herbst 
have  shown  that  the  belated  sperm-nucleus  may 
unite  with  one  of  the  two  nuclei  that  result  from  the 
first  division  of  the  egg  nucleus. 

On  the  other  hand,  the  second  case  of  the  fly  (where 
only  a  small  part  of  the  body  is  male)  may  be  better 
accounted  for  by  my  hypothesis.  It  is  known  that 
single  sperms  that  enter  an  egg  without  a  nucleus, 
or  even  with  one,  may  divide.  The  two  hypotheses 
are  not  mutually  exclusive,  but  rather  supplementary. 

Toyama  has  described  a  gynandromorph  in  the 
silkworm  that  arose  in  a  cross  between  a  race  with  a 
banded  caterpillar  (the  female  parent)  and  a  race 
with  a  white  caterpillar  (the  male  parent).  As  shown 
in  Fig.  84,  the  gynandromorph  was  banded  on  the  left 
(maternal)  side  and  white  on  the  other  (right)  side. 
When  the  adult  moth  emerged,  the  left  side  was  female 
and  right  side  was  male.  Since  the  sperm  alone  bore 
the  white  character,  which  is  a  recessive  character,  it 
appears  that  the  right  side  must  have  come  from  sperm 
alone.  This  is  in  accordance  with  my  hypothesis,  but 
I  have  also  shown  that  Gynandromorphs  may  arise 
through  somatic  dislocations  of  the  sex  chromosomes 
in  the  early  embryo.  Gynandromorphs  are  not  un- 
common in  insects,  rare  (or  never  present)  in  birds  and 
mammals. 

The  explanation  K  f  this  difference  is  found,  I  think,  in 


166 


HEREDITY  AND   SEX 


*  * 


FIG.  84.  —  I,  a,  plain,  6,  striped  caterpillar  of  silkworm.  II,  a,  gynandro- 
morph  silkworm,  b,  moth  of  same.  Ill,  wings  of  last.  IV,  dorsal  view  of 
same  moth.  V,  abdomen  of  same.  VI,  end  of  abdomen  of  same  moth 
VII,  normal  female,  and  VIII,  a  normal  male.  (After  Toyama.) 


HERMAPHRODITISM  167 

the  relation  of  the  secondary  sexual  characters  to  the 
sex  glands.  In  insects  the  characters  in  question  are  not 
dependent  on  the  presence  or  absence  of  these  glands. 
Hence,  when  such  conditions  occur  after  fertilization, 
as  those  I  have  just  considered,  each  part  may  develop 
independently  of  the  rest. 

HERMAPHRODITISM 

In  almost  all  of  the  great  groups  of  animals  a  condi- 
tion is  found  in  which  complete  sets  of  ovaries  and  testes 
occur  in  the  same  individual.  This  condition  is  called 
"hermaphroditism."  In  some  groups  of  animals,  as  in 
flatworms,  leeches,  mollusks,  hermaphroditism  is  the 
rule,  and  it  is  the  common  condition  in  flowering 
plants.  Sometimes  there  is  only  one  system  of  outlets 
for  eggs  and  sperm,  but  not  infrequently  each  has  a 
separate  system. 

Here  there  is  no  problem  of  the  production  of  males 
and  females,  for  one  kind  of  individual  alone  exists. 
But  what  determines  that  in  one  part  of  the  body 
male  organs  develop,  and  in  another  part  a  female 
system  ? 

Two  views  suggest  themselves,  either  somatic  segre- 
gation, or  regional  differentiation.  By  somatic  seg- 
regation I  mean  that  at  some  time  in  the  development 
of  the  embryo  —  at  some  critical  division  —  a  separa- 
tion of  chromosomes  takes  place  so  that  an  egg-produc- 
ing group  and  a  sperm-producing  group  is  formed. 
There  is  no  direct  evidence  in  support  of  this  view. 

Another  view  is  that  the  formation  of  ovary  and 
testis  is  brought  about  in  the  same  way  as  all 
differentiations  of  body  organs,  as  for  example  the 


168 


HEREDITY  AND   SEX 


formation  of  liver  and  lungs  and  pancreas  from  the 
digestive  tract.  The  following  case  may  perhaps 
be  considered  as  supporting  such  an  hypothesis.  In 
a  hermaphroditic  worm,  Criodrilus  lacuum  the  ovaries 
lie  in  the  thirteenth  and  the  testes  in  the  tenth  and 
eleventh  segments.  If  the  anterior  end  be  cut  off,  a 
new  one  regenerates,  as  shown  by  Janda  (Fig.  85), 


Fig.  1. 


FIG.  85.  —  1,  anterior  end  of  normal  criodrilus,  showing  reproductive 
system;  2-5,  regenerated  anterior  ends.     (After  Janda.) 

in  which  the  ovaries  and  testes  reappear  approximately 
in  their  appropriate  regions.  It  is  true  their  location 
is  more  liable  to  vary  than  in  the  normal  worm,  but 
this  is  unimportant. 

In  the  Gephyrean  worm,  Bonellia  viridis,  the  degen- 
erate male  is  parasitic  on  the  female.  Baltzer  has  dis- 
covered that  every  fertilized  egg  is  potentially  a  male 
or  a  female.  For,  if  the  swimming  larvae  have  a  chance 


HERMAPHRODITISM  169 

to  settle  down  on  an  adult  bonellia  they  become  males ; 
but  if  the  larvae  have  no  such  opportunity  a  long  period 
without  further  development  intervenes,  and  later  the 
larvae  become,  for  the  most  part,  females,  a  few 'become 
males,  and  a  few  hermaphrodites.  Bonellia  appears 
.therefore  to  be  a  protandric  hermaphrodite,  —  like  many 
plants.  Somewhat  similar  relations  are  known  for  Hydra 
viridis,  as  shown  by  Nussbaum,  and  by  Whitney. 

In  cases  where  a  sexual  generation  alternates  with 
a  hermaphroditic  generation,   the  problem  of  the  two 


FIG.  86.  —  Rhabditis  nigrivenosa,  male  (left)  and  female  (right).     (After 

Leunis.) 

sexes  reappears.  There  is  but  one  case  in  animals 
that  has  been  adequately  worked  out.  A  nematode 
worm,  Rhabditis  nigrovenosa,  lives  as  a  parasite  in 
the  lungs  of  frogs.  It  is  an  hermaphrodite.  Its 
eggs  give  rise  to  another  generation  that  lives  in  mud 
and  slime.  In  this  generation  two  kinds  of  individuals 
are  present  —  true  males  and  females  (Fig.  86).  The 
females  produce  eggs,  that  are  fertilized,  and  develop 


170 


HEREDITY  AND   SEX 


into  the  hermaphrodites  which  find  their  way  again 
into  the  lungs  of  frogs. 

Boveri  and  Schleip  have  worked  out  the  history 
of  the  chromosomes  in  this  case.     The  cells  of  the 


FIG.  87.  —  Chromosomes  of    Angiostomum.     (A),    oogonia,    CB),  equa- 
torial   plate     of    first    maturation    division;     (C),    young    spermatocyte 
(D) ,  first  spermatocyte  division  in  metaphase ;     (E) ,  same  in  anaphase 
(F),  spermatocyte  of  second  division;     (G),   and   (#),  division  of    same 
(/),  and  (K),  loss  of  X  at  plane  of  division ;    (L),  first  segmentation  division 
of  a  male  embryo ;    two  sets  of  chromosomes  (5  and   6=11  respectively) 
separate ;     (M )   equatorial  plate  of  dividing  cell  of    female  embryo  =  12 
chromosomes ;    (N) ,  same  from  male  embryo  =11  chromosomes.      (After 
Schleip.) 

hermaphrodite  have  twelve  chromosomes  (Fig.  87). 
The  eggs,  after  extruding  two  polar  bodies,  have 
six  chromosomes.  The  spermatozoa  that  develop 
in  the  body  of  the  same  animal  have  six  or  five  chro- 
mosomes each,  because  one  chromosome  is  lost  in  half 


HERMAPHRODITISM  171 

of  the  cells  by  being  left  at  the  dividing  line  between 
the  two  cells.  We  can  understand  how  two  kinds 
of  individuals  are  produced  by  the  hermaphrodites 
from  the  two  classes  of  sperm  combining  at  random 
with  the  eggs. 

These  two  kinds  of  individuals  are  females  with 
twelve  chromosomes,  and  males  with  eleven  chromo- 
somes. How  then  can  we  get  back  to  the  hermaph- 
roditic generation?  Boveri  and  Schleip  suggest  that 
the  males  again  produce  two  kinds  of  spermatozoa,  - 
they  have  shown  this  to  be  the  case  in  fact,  —  and  that 
the  male-producing  spermatozoa  become  function- 
less.  Here  we  have  at  least  an  outline  of  some  of 
the  events  in  the  life  cycle  of  this  worm  in  relation 
to  the  chromosomes,  but  no  explanation  of  hermaph- 
roditism. 

Turning  to  plants,  there  are  the  interesting  experi- 
ments of  the  Marchals  with  mosses.  They  show  that 
the  sporophyte  is  hermaphroditic  and  has  the  factors 
for  maleness  and  femaleness  combined  as  a  result 
of  fertilization ;  while  in  the  formation  of  the  spores 
the  factors  in  question  are  separated. 

Blakeslee  has  found  somewhat  similar  relations  in 
certain  of  the  molds.  The  spores  in  molds  contain 
more  than  one  nucleus,  therefore  it  is  not  clear  how 
segregation  in  the  sense  used  for  other  cases  applies 
here. 

In  the  flowering  plants  that  are  hermaphroditic 
we  have  Correns'  experiments,  in  which  he  crossed  an 
hermaphroditic  type  of  Bryonia  alba  with  a  type 
B.  dioica  in  which  the  sexes  are  separate.  The 
cross  when  made  one  way  gives  only  females,  while 


172  HEREDITY  AND  SEX 

the  reciprocal  cross  gives  males  and  females  in  equal 
numbers.  Correns'  interpretation  is  shown  in  the 
lower  part  of  the  next  diagram. 

Bryonia  dioica  and  B.  alba 
B.  dioica  $    by    B.  alba  $        B.  alba  9    by    B.  dioica  $ 

\        /  \        / 

\  /  \  / 

Females  Females  and  Males 

Correns1  Explanation 

F  -  F       B.  dioica  £  (FM)-(FM}  B.  alba  9 

(FM  )—  (FM  )  B.  alba  $  F  -  M      B.  dioica  $ 

F(FM)    female  F(FM)    female 

male 


It  is  based  in  the  first  case  on  the  assumption  that 
the  hermaphroditic  condition  of  B.  alba  is  recessive  to 
the  dioecious  condition  of  B.  dioica,  and  that  the  female 


tlialeTf.         -HcrmabH.  FH 


i>—  H 


f  H     fwotv.  -PH       Txemv. 

"f" —  H    Kerriv.  ovtile 
'F  —  -p    mate 


FIG.  88.  —  Diagram  to  illustrate  G.  H.  Shull's  results  on  Lychnis  dioica. 
The  symbols  here  used  are  not  those  used  by  Shull.  Two  types  are  assumed 
not  to  appear,  viz.  HH  and  Hf.  Third  cross  should  give  FH  also. 


PARTHENOGENESIS  1 73 

dioica  is  homozygous  for  the  sex  factor.  The  recip- 
rocal cross  is  explained  on  the  basis  that  maleness 
dominates  femaleness.  The  sex-determining  factors 
must  here  be  different  from  cases  like  the  insects. 

Shull  obtained  as  a  mutant  a  hermaphroditic  plant 
of  Lychnis  dioica.  The  next  diagram  (Fig.  88)  gives 
the  principal  facts  of  his  crosses.  When  a  female 
plant  is  fertilized  by  the  pollen  of  the  hermaphrodite, 
two  kinds  of  offspring  are  produced  —  females  and 
hermaphrodites.  When  the  hermaphrodite  is  self- 
fertilized,  the  same  two  classes  are  produced.  When 
the  ovule  of  the  hermaphrodite  is  fertilized  by  the 
pollen  from  the  male  plant,  two  kinds  of  offspring 
are  again  produced  —  female  and  male.  ShmTs  inter- 
pretation is  too  involved  to  give  here.  In  the  diagram 
the  scheme  is  worked  out  on  the  purely  arbitrary 
scheme  that  the  hermaphrodite  is  FH,  in  which  F 
is  a  female  factor,  and  H  a  modification  of  it  which 
gives  hermaphroditism.  This  leads  to  the  further 
assumption  that  ovule  and  pollen,  bearing  the  H 
factor,  cannot  produce  a  plant  nor  can  the  combination 
/  H.  This  scheme  is  only  intended  as  a  shorthand  way 
of  indicating  the  results,  and  not  as  an  interpretation 
of  actual  conditions. 

PARTHENOGENESIS 

A  third  important  condition  in  which  the  heredity 
of  sex  is  involved  is  found  in  parthenogenesis. 

It  has  long  been  known  to  biologists,  that  in  many 
different  species  of  animals  and  plants  eggs  develop 
without  being  fertilized.  This  is  recognized  as  a 
regular  method  of  propagation  in  some  species.  The 


174  HEREDITY  AND  SEX 

eggs  are  produced  in  the  same  way  as  are  other  eggs. 
They  are  produced  in  ovaries  that  have  the  same 
structure  as  the  ovaries  that  give  rise  to  ordinary 
eggs.  Parthenogenetic  eggs  differ  from  spores,  not 
only  in  their  origin  in  an  ovary,  but  in  that  they  also 
produce  polar  bodies  like  ordinary  eggs.  Most,  but 
not  all,  parthenogenetic  eggs  give  rise,  however,  to 
only  one  polar  body.  Some  of  them  at  least  fail  to 
pass  through  the- stage  of  synapsis,  and,  in  consequence, 
they  retain  the  full  number  of  chromosomes. 


e  ;'^«.. 


FIG.  89.  —  Miastor,  sexual  male  and  female  (to  right).     Three  larvae 
with  young  inside  (to  left). 

A  few  examples  will  bring  the  main  facts  before  us. 

A  fly,  miastor,  appears  in  the  spring  of  the  year 
under  two  forms,  male  and  female  (Fig.  89) .  The  eggs 
are  fertilized  and  each  produces  a  worm-like  larva. 
This  larva  produces  eggs  while  still  in  the  larval  stage. 
The  eggs  develop  without  fertilization,  and  produce 
new  larvae,  which  repeat  the  process.  This  method 
of  propagation  goes  on  throughout  the  rest  of  the 
year  until  finally  the  adult  winged  flies  reappear. 

The  bee  is  the  most  remarkable  instance,  for  here 


PARTHENOGENESIS  175 

the  same  egg  will  produce,  if  it  is  fertilized,  a  female 
(queen  or  worker),  or,  if  it  is  not  fertilized,  a  male 
(drone).  If  the  queen  deposits  an  egg  in  a  cell  of  the 
comb  that  has  been  built  for  a  queen  or  a  worker,  she 
fertilizes  the  egg ;  if  in  a  drone  cell,  the  egg  is  not  fertil- 
ized. We  need  not  conclude  that  the  queen  knows 
what  she  is  about  —  the  difference  in  shape  of  the  drone 
cell  may  suppress  the  reflex,  that  in  the  other  cases 
sets  free  the  sperm. 

The  case  of  the  bee  has  attracted  so  much  attention 
that  I  may  be  allowed  to  pause  for  a  moment  to  point 
out  some  of  the  most  recent  results  connected  with  the 
formation  of  the  germ-cells. 

The  egg  produces  two  polar  bodies  —  the  process 
being  completed  after  the  sperm  has  entered  the  fer- 
tilized egg  (Fig.  90).  Eight  chromosomes  are  present 
at  each  division.  Eight  remain  in  the  egg  (these  are 
double  chromosomes  —  therefore  16).  The  sperm 
brings  in  8  (double)  chromosomes  so  that  the  female 
comes  to  have  16  single  chromosomes  in  her  cells.  There 
is  only  one  kind  of  spermatozoon,  as  shown  by  the  figure, 
for  the  first  spermatocyte  division  is  abortive  —  all 
the  chromosomes  passing  into  one  cell  only,  and  the 
second  division  gives  rise  to  a  small  cell,  that  does  not 
produce  a  spermatozoon,  and  a  large  cell  that  becomes 
a  spermatozoon. 

If  the  egg  is  not  fertilized,  it  also  gives  off  two  polar 
bodies.  It  has  8  chromosomes  left.  The  male  de- 
velops with  the  half  number.  The  formula  for  the 
female  will  be  XABCD  XABCD  and  for  the  male 
XABCD. 

If  the  bee  conforms  to  the  ordinary  type  for  insects, 


176  HEREDITY  AND  SEX 

we  may  suppose  that  one  sex  chromosome  is  present 
in  the  male  or  at  least  one  differential  factor  for  sex, 
and  that  it  is  present  in  all  the  functional  spermato- 
zoa. The  female  will  then  have  two  such  chromo- 
somes and  come  under  the  general  scheme  for  insects. 


16  <? 


FIG.  90.  —  Oogenesis  and  spermatogenesis  in  bee.  Four  upper  figures, 
A-D,  show  formation  of  first  (A),  and  second  (B)  polar  bodies.  Only  inner 
group  of  chromosomes  remains  (C)  to  form  egg  nucleus.  Entrance  of  sperm 
nucleus  in  D.  E  shows  scheme  of  these  two  divisions  involving  eight  double 
(82)  chromosomes.  F,  first  and  second  spermatocyte  divisions,  the  first, 
a,  b,  abortive,  leading  to  pinching  off  of  a  small  cell  without  a  nucleus,  the 
second,  c,  c,  leading  to  formation  of  a  large  (functional)  and  an  abortive 
cell  (above). 

In  the  gall  fly,  Neuroterus  lenticularis,  partheno- 
genetic  females  appear  early  in  the  spring.  Their  eggs 
produce  females  and  males  —  the  second  generation. 
The  fertilized  eggs  of  these  females  give  rise  the  follow- 
ing year  to  the  spring  parthenogenetic  females.  Don- 
caster  has  found  that  each  parthenogenetic  female 


PARTHENOGENESIS 


177 


produces  eggs,  all  of  which  give  rise  to  females  or  else 
to  males.  In  connection  with  this  fact  he  finds  that 
the  eggs  of  some  females  do  not  give  off  any  .polar 
bodies  but  retain  the  full  number  (20)  of  chromosomes. 


ft* 


FIG.  91.  —  Illustrating  chromosome  cycle  in  Neuroterus.  A,  one  type  of 
-spring  female,  whose  eggs  (containing  20  chromosomes)  produce  no  polar 
bodies.  Only  sexual  females  result.  B,  the  other  type  of  spring  female 
whose  eggs  form  two  polar  bodies,  leaving  10  chromosomes  in  egg.  These 
eggs  give  rise  to  males.  C,  ripening  of  egg  of  sexual  female  (2d  generation), 
and  D,  spermatogenesis  of  male  (second  generation). 

These  eggs  produce  sexual  females  (in  left-hand  side 
of  Fig.  91).  From  the  eggs  of  other  parthenogenetic  fe- 
males two  polar  bodies  are  given  off,  and  the  half  (10) 
number  of  chromosomes  is  left  in  the  egg  (see  right-hand 
side  of  Fig.  91).  These  eggs  produce  males.  The  life 


178 


HEREDITY  AND  SEX 


cycle  finds  its  explanation  in  these  relations  except  that 
the  origin  of  the  two  kinds  of  parthenogenetic  females 
is  unexplained.  If  we  were  justified  in  assuming  that 
two  classes  of  female-producing  sperm  are  made  in  the 
male,  even  this  point  would  be  cleared  up,  for  in  this 


FIG.  92.  —  Life  cycle  of  Phylloxera  carycecaulis. 

way  the  two  classes  of  parthenogenetic  females  could 
be  explained. 

In  another  group  of  insects,  the  aphids  and  phyllox- 
erans,  the  situation  is  different. 

In  the  phylloxerans  of  the  hickories  there  emerges 
in  the  spring,  from  a  fertilized  egg,  a  female  known  as 
the  stem  mother  (Fig.  92).  She  pierces  a  young  leaf 


PARTHEXOGENESIS  179 

with  her  proboscis,  which  causes  a  proliferation  of  the 
cells  of  the  leaf.  Eventually  the  leaf  cells  grow  so  fast 
that  the  stem  mother  is  overarched  in  the  gall  that  she 
has  called  forth. 

Inside  the  gall  she  begins  to  lay  her  eggs.  From  these 
eggs  emerge  young  individuals  that  remain  in  the  gall 
until  they  pass  their  last  molt,  when  they  become  winged 
migrants.  Externally  all  the  migrants  are  alike;  but 
if  they  are  dissected,  it  will  be  found  that  some  of  them 
have  large  eggs,  some  small  eggs.  But  all  the  offspring 
of  the  same  mother  are  of  one  or  of  the  other  sort. 

The  migrants  crawl  out  of  the  opening  in  the  gall  and 
fly  away.  Alighting  on  other  hickories,  they  quickly 
•deposit  their  eggs.  From  the  large  eggs  the  sexual 
females  emerge.  They  never  grow  any  bigger  than  the 
egg  from  which  they  hatched.  In  fact,  they  have  no 
means  of  feeding,  and  contain  only  one  large  egg  with 
a  thick  coat  —  an  egg  almost  as  large  as  the  female 
herself. 

From  the  small  eggs  of  the  migrants,  minute  males 
are  produced  —  ripe  at  their  birth.  They  fertilize 
the  sexual  female.  She  then  deposits  her  single  egg  on 
the  bark  of  the  hickory  tree.  From  this  egg  (that  lies 
dormant  throughout  the  entire  summer  and  following 
winter)  there  emerges  next  spring  a  female,  the  stem 
mother  of  a  new  line. 

Here  we  find  three  generations  in  the  cycle  —  two 
of  which  reproduce  by  parthenogenesis.  The  first 
parthenogenetic  generation  gives  rise  to  two  kinds  of 
individuals  —  one  makes  large  eggs,  the  other  small 
eggs.  The  large  eggs  produce  sexual  females,  the  small 
eggs  males. 


180  HEREDITY  AND  SEX 

A  study  of  the  chromosomes  has  explained  how  some 
of  these  changes  in  successive  generations  are  brought 
about.  It  has  explained,  for  instance,  how  males  are 
produced  by  parthenogenesis,  and  why  the  sexual  egg 
produces  only  females.  Let  us  take  up  the  last  point 
first. 

When  the  spermatocytes  are  produced,  we  find,  as  in 
many  other  insects,  that  at  one  division  a  sex  chromo- 
some passes  to  one  cell  only  (Fig.  93) .  Two  classes  of 
cells  are  produced  —  one  with  three,  one  with  two, 
chromosomes.  The  latter  degenerates,  and  in  conse- 
quence only  the  female-producing  spermatozoa  become 
functional.  All  fertilized  eggs  give  rise  therefore  to 
females. 

The  second  point  that  has  been  made  out  concerns 
the  production  of  the  male.  When  the  small  egg 
produces  its  single  polar  body,  all  of  the  chromosomes 
divide,  except  one,  which  passes  out  entire  into  the 
polar  body.  In  consequence  the  number  of  chromo- 
somes left  in  the  egg  is  one  less  than  the  total  number. 
In  a  word,  there  are  five  chromosomes  in  the  male, 
while  there  are  six  chromosomes  in  the  female  (Fig.  93^. 
By  throwing  out  one  chromosome,  the  change  is  effected. 
The  chromosome  is  the  mate  of  the  sex  chromosome, 
that  appeared  as  a  lagging  chromosome  in  the  spermato- 
genesis. 

In  the  large  egg  no  such  diminution  takes  place, 
consequently  the  diploid  number  of  chromosomes  is 
present  in  the  female.  These  unite  in  pairs  and  are 
reduced  to  three  when  the  two  polar  bodies  of  the 
sexual  egg  are  produced. 

We  see  that  by  means  of  the  chromosomes  we  can 


PARTHENOGENESIS 


181 


bring  this  case  into  line  with  the  rest  of  our  informa- 
tion bearing  on  the  relation  of  the  chromosomes  to  sex. 
One  important  point  still  remains  to  be  explained. 
What  causes  some  of  the  migrants  to  produce  large 

PHYLLOXERA    CAKWECAUIJS 


7>lcct* 

9?  tern,  7?lcthv&    -C 


O 


o 


O 


9  f? 


O 


o 


o 


FIG.  93.  —  Chromosomal  cycle  of  P.  carycecaulis. 


182  HEREDITY  AND  SEX 

v- 

eggs  and  others  small  eggs  ?  There  must  be  either  two 
kinds  of  stem  mothers  or  one  kind  with  double  po- 
tentiality. Inasmuch  as  in  other  parthenogenetic  types 
there  is  experimental  evidence  to  prove  that  environ- 
mental conditions  determine  which  alternative  state, 
whether  male-producing  or  female-producing  individ- 
ual, is  realized,  so  here  we  may,  provisionally,  follow 
the  same  interpretation.  Once  the  course  is  deter- 
mined the  subsequent  internal  events  follow  for  two 
generations  in  a  definite  order.  If  the  stem  mother  has 
been  affected  in  one  way,  all  of  her  daughters  produce 
large  eggs ;  if  in  the  other  way,  small  eggs. 

In  another  group  of  animals,  the  daphnians,  parthen- 
ogenetic species  occur,  that,  in  certains  respects,  are 
like  the  phylloxerans ;  but  these  species  illustrate  also 

\   another  relation  of  general  interest. 

*  The  fertilized  winter  egg  produces  always  a  female, 
the  stem  mother,  which  gives  rise  by  parthenogenesis 
to  offspring  like  herself,  and  the  process  may  continue 
a  long  time.  Each  female  produces  one  brood,  then 
another  and  another.  The  last  broods  fail  to  develop, 
and  this  is  a  sign  that  the  female  has  nearly  reached 
the  end  of  her  life. 

But  a  parthenogenetic  female  may  produce  one  or  two 
large  resting  eggs  instead  of  parthenogenetic  females, 
and  the  same  female  may  at  another  time  produce  a 
brood  of  males.  The  large  resting  eggs  are  inclosed 
in  a  thick  outer  protecting  case.  They  must  be  fer- 
tilized in  order  to  develop,  yet  they  do  not  develop  at 
once,  but  pass  through  an  enforced,  or  a  resting,  stage 
that  may  be  shortened,  if  the  egg  is  dried  and  then 
returned  to  water. 


PARTHENOGENESIS 


183 


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FIG.  94.  —  Life  cycle  of  Simocephalus ;    successive  broods  in  horizontal 
lines,  successive  generations  in  vertical  lines.     (After  Papanicolau.) 


184  HEREDITY  AND  SEX 

In  this  life  history  we  do  not  know  what  changes 
take  place  in  the  chromosomes.  It  has,  however,  often 
been  claimed  in  this  case  that  the  transition  from  par- 
thenogenesis to  sexual  reproduction  is  due  to  changes  in 
the  environment. 

In  fact,  this  is  one  of  the  stock  cases  cited  in  the  older 
literature  to  show  that  sex  is  determined  by  external 
agents.  It  was  said,  that  if  the  environment  causes 
males  to  appear,  then  sex  is  determined  by  the  environ- 
ment. But  as  a  matter  of  fact,  in  so  far  as  changes  in 
the  environment  affect  this  animal,  they  cause  it  to 
cease  reproducing  by  parthenogenesis,  and  induce  sexual 
reproduction  instead.  The  evidence  is  consistent  in 
showing  that  any  external  change  that  affects  the 
mode  of  reproduction  at  all  calls  forth  either  sexual 
eggs  or  males.  The  machinery  of  parthenogenesis 
is  switched  off,  and  that  for  sexual  reproduction  is 
turned  on. 

The  discrepancies  that  appear  in  the  older  accounts 
are  probably  due,  as  Papanicolau  suggests,  to  dif- 
ferent observers  using  females  that  belong  to  different 
phases  of  the  parthenogenetic  cycle.  Papanicolau, 
starting  in  each  case  with  a  winter  egg,  finds  that  as 
successive  broods  are  produced  the  color  of  the  par- 
thenogenetic eggs  can  be  seen  to  undergo  a  progressive 
change  from  blue  to  violet.  As  the  change  progresses 
the  chance  that  males  and  sexual  eggs  ("  females  ")  will 
appear  is  greater.  Until  towards  the  end  of  the  life  of 
the  individual  the  males  and  females  come,  as  it  were, 
of  themselves  (Fig.  94).  If,  however,  individuals  of 
successive  broods  are  subjected  to  cold,  it  is  found  that 
while  earlier  broods  do  not  respond,  later  ones  respond 


PARTHENOGENESIS  1 85 

more  and  more  easily  and  change  over  to  the  sexual 
phase  of  the  cycle. 

What  has  just  been  said  about  the  successive  broods 
might  be  said  equally  of  the  first-born  offspring  of  the 
successive  generations,  as  Papanicolau's  table  shows 
(Fig.  94).  Later  born  offspring  respond  more  readily 
than  do  those  that  are  historically  nearer  to  the  fer- 
tilized egg. 

More  recently  Grosvenor  and  Smith  have  found  for 
Moina  that  if  females  are  reared  alone  (at  25°-30°  C.) 
no  sexual  broods  appear,  while  parallel  cultures  of 
females  crowded  together  give  30  or  higher  per  cents 
of  males.  Agar  has  carried  isolated  females  through  46 
parthenogenetic  generations  and  has  found  that  mothers 
from  late  brood  also  give  only  parthenogenetic  offspring 
in  a  suitable  environment. 

A  third  type,  Hydatina  senta  (Fig.  95),  an  almost 
microscopic  wormlike  animal  belonging  to  the  rotifers, 
reproduces  by  parthenogenesis. 

The  resting  egg  always  gives  rise  to  a  parthenogenetic 
female,  i.e.  she  also  reproduces  by  parthenogenesis. 
Whitney  has  obtained  500  generations  produced  in  this 
way.  But  from  time  to  time  another  kind  of  individual 
appears.  She  is  externally  like  the  parthenogenetic 
female,  but  has  entirely  different  capacities.  Her 
eggs  may  be  fertilized,  and  if  they  are  they  become 
resting  eggs  inclosed  in  a  hard  case.  The  sperm  enters 
when  the  eggs  are  immature  and  still  in  the  ovary  of 
the  mother.  The  presence  of  a  spermatozoon  in  an  egg 
determines  that  the  egg  goes  on  to  enlarge  and  to  pro- 
duce its  thick  coat.  But  if  perchance  no  males  are 
there  to  fertilize  the  eggs,  this  same  female  produces  a 


186 


HEREDITY  AND   SEX 


crop  of  male  eggs  that  develop  into  males  without 
being  fertilized  at  all. 

There  are  several  facts  of  unusual  interest  in  the 


HYDAT/MA  SEMTA 


Male 


FIG.  95.  —  Life  cycle  of  Hydatina  senta. 

life  history  of  hydatina,  but  we  have  occasion  to  consider 
only  one  of  them.  It  has  been  claimed  in  this  case 
also  that  external  conditions  determine  the  production 
of  males.  A  more  striking  example  of  the  erroneous- 


PARTHENOGENESIS  187 

ness  of  this  general  conclusion  would  be  hard  to  find ; 
for,  in  the  first  place,  as  we  have  seen,  the  same  indi- 
vidual that  produces  males  will  produce  out  of  the  same 
eggs  females  if  she  happens  to  be  fertilized.  In 
the  second  place  the  older  evidence  which  was  supposed 
to  establish  the  view  that  certain  specified  changes  in 
the  environment  cause  the  production  of  males  has 
been  overthrown. 

The  French  zoologist,  Maupas,  is  deserving  of  high 
praise  for  working  out  some  of  the  most  essential  facts 
in  the  life  cycle  of  hydatina,  and  for  opening  up  a 
new  field  of  investigation.  But  the  evidence  which 
he  brought  forward  to  show  that  by  a  low  tempera- 
ture a  high  production  of  males  is  caused  has  not 
been  confirmed  by  very  careful  and  extensive  repeti- 
tion of  his  experiments  by  Whitney  and  by  A.  F. 
Shull.  The  evidence  that  Nussbaum  obtained  which 
seemed  to  him  to  show  that  food  conditions  de- 
termined the  production  of  males  has  likewise  not 
borne  the  test  of  more  recent  work  by  Punnett,  Shull, 
and  Whitney. 

It  has  been  found,  however,  that  the  production  of 
the  sexual  phase  of  the  cycle  can  be  suppressed  so  that 
the  animals  continue  almost  indefinitely  propagating 
by  parthenogenesis.  In  several  ways  this  may  be 
accomplished.  If  hydatina  is  kept  in  a  concentrated 
solution  of  the  food  culture,  the  sexual  phase  does  not 
appear.  The  result  has  nothing  to  do  with  the  abun- 
dance of  food,  for,  if  the  food  be  filtered  out  from 
the  fluid  medium,  the  filtrate  gives  the  same  result. 
The  following  table  given  by  Shull  shows  this  very 
clearly. 


188 


HEREDITY  AND    SEX 


SPRING  WATER 

OLD  CULTURE  FILTRATE 

One-fourth 

One-half 

Three-fourths 

Undiluted 

cf   ? 

9   9 

<J    ? 

?  9 

d1    ? 

?  9 

cf  9 

9  9 

d  9 

9  9 

26 

177 

25 

407 

15 

350 

8 

362 

0 

337 

%ofc?9 

12.8 

5.7 

4.1 

2.1 

0.0 

Showing  the  number  of  male-  and  female-producers  in  the  progeny 
of  five  sister  individuals  of  Hydatina  senta,  one  line  being  reared  in 
spring  water,  the  others  in  various  concentrations  of  the  filtrate 
from  old  food  cultures. 


Mitchell  found  that  a  sudden  change  in  food  causes 
male-producing  individuals  to  appear  in  Asplanchna; 
and  Whitney  can  now  produce,  at  will,  in  Hydatina  a 
high  percentage  of  male  producers.  Females  feeding 
on  the  colorless  flagellate  Polytoma  were  fed  on  the 
green  flagellate  Duniella,  and  gave  birth  to  80  %  of 
male  producers ;  while  the  control  female  fed  on  Poly- 
toma gave  birth  to  only  9  %  of  male  producers.  Since 
the  eggs  of  the  male  producer  give  rise  to  sexual  fe- 
males, or  to  males,  according  to  whether  they  are  fer- 
tilized or  whether  they  are  not,  sex  itself  is  not  here 
determined  by  the  environment,  but  by  fertilization ; 
the  environment  determines  the  kind  of  reproduction. 


ARTIFICIAL    PARTHENOGENESIS 

We  have  now  considered  some  of  the  most  striking 
examples  of  natural  parthenogenesis  in  the  animal 
kingdom.  The  facts  show  that  fertilization  of  the  egg 
is  not  in  itself  essential  for  development.  The  in- 


ARTIFICIAL  PARTHENOGENESIS  189 

dividuals  that  develop  from  parthenogenetic  eggs  are  as 
vigorous  as  those  from  eggs  that  have  been  fertilized. 
We  have  seen  that  such  eggs  without  being  fertilized 
are  capable  of  producing  sexual  females  and  males. 
In  one  case,  at  least,  we  have  seen  how  the  process  is 
accomplished. 

When  we  review  the  facts  of  natural  parthenogenesis, 
we  find  certain  relations  that  arrest  our  attention. 

Most  parthenogenetic  eggs  give  off  only  a  single 
polar  body,  while  fertilized  eggs  without  exception  give 
off  two  polar  bodies.  This  difference  is  clearly  con- 
nected with  the  fact  that  in  parthenogenetic  eggs  the 
full  number  or  diploid  number  of  chromosomes  is  re- 
tained by  the  egg.1  In  fertilized  eggs  half  the  chromo- 
somes are  thrown  out  in  one  of  the  two  polar  bodies. 
The  number  is  made  good  by  the  chromosomes  brought 
in  by  the  spermatozoon. 

But  this  difference  does  not  in  the  least  explain  nat- 
ural parthenogenesis ;  for  we  have  experimental  evi- 
dence to  show,  that  an  egg  will  develop  when  only  half 
the  number  of  chromosomes  is  present  —  one  set  will 
suffice. 

There  is  another  fact  about  parthenogenetic  eggs 
that  has,  I  believe,  been  generally  overlooked.  Many 
of  these  eggs  begin  to  develop  into  an  embryo  before 
they  reach  the  full  size  of  the  fertilized  eggs  of  the 
same  species.  This  is  true  at  least  of  the  eggs  of  aphids, 
phylloxerans,  daphnians,  and  rotifers.  I  interpret  this 

1  According  to  my  observations  on  aphids  and  phylloxerans,  the 
synapsis  stage  is  omitted  in  parthenogenetic  eggs,  hence  there  is 
no  union  (or  reduction)  of  the  chromosomes.  The  omission  of  this 
stage  may  have  something  to  do  with  parthenogenesis,  although  it 
is  not  evident  what  the  relation  may  be. 


190  HEREDITY  AND  SEX 

to  mean  that  the  eggs  begin  their  development  be- 
fore there  has  been  produced  over  their  surface  a 
layer  that  in  the  mature  egg  seems  to  have  an  im- 
portant influence  in  restraining  sexual  eggs  from  de- 
velopment. 

This  brings  us  at  once  to  a  consideration  of  what 
keeps  sexual  eggs  from  developing  until  they  are  fer- 
tilized. 

In  recent  years  a  great  variety  of  methods  has  been 
discovered  by  means  of  which  sexual  eggs  can  be  made 
to  develop  without  fertilization.  This  process  is 
called  artificial  parthenogenesis.  We  owe  especially 
to  Professor  Jacques  Loeb  the  most  successful  accom- 
plishment of  this  important  feat.  The  discovery  in 
his  hands  has  led  to  very  great  advances  in  our 
understanding  of  the  developmental  process. 

The  chief  importance  of  Loeb's  work  lies,  in  my 
opinion,  not  only  in  the  production  of  embryos  with- 
out fertilization  (nature  has  long  been  conversant 
with  such  methods),  but  in  other  directions  as  well. 

First,  it  has  thrown  light  on  the  nature  of  the  in- 
hibitory process  that  holds  back  the  sexual  egg  from 
developing  until  the  sperm  enters. 

Second,  the  information  gained  in  this  way  tells  us 
something  of  how  the  sperm  itself  may  act  on  the  egg 
and  start  it  on  its  course. 

Third,  it  opens  up  the  opportunity  of  studying  cer- 
tain problems  connected  with  the  determination  of  sex 
that  can  be  gained  in  no  other  way. 

Let  me  attempt  briefly  to  elaborate  some  of  these 
points. 

In  many  eggs,  perhaps  in  all,  a  membrane  is  produced 


ARTIFICIAL  PARTHENOGENESIS  191 

at  the  surface  of  the  egg  immediately  after  the 
sperm  has  entered.  Here  we  have  ocular  evidence 
that  fertilization  effects  a  change  in  the  surface  layer 
of  the  egg. 

It  has  been  shown  that  after  this  membrane  is  formed, 
the  permeability  of  the  egg  to  salts  and  other  agents  is 
affected  and  that  the  processes  of  oxidation  are  greatly 
accelerated. 

In  other  words,  the  ulterior  of  the  unfertilized  egg  is 
separated  by  means  of  its  membrane  from  many  things 
in  the  surrounding  medium  —  oxygen  and  the  salts  hi 
sea-water,  for  example.  The  egg  after  fertilization 
lives  hi  a  new  world. 

These  same  changes  are  brought  about  by  those 
external  agents  that  cause  artificial  parthenogenesis. 
Loeb  has  shown  by  a  thorough  study  of  the  conditions 
that  any  substance  that  causes  cytolysis  (a  typical  form 
of  cell  destruction)  will  induce  parthenogenesis  if  ap- 
plied to  the  surface  of  the  egg  only. 

Loeb  has  shown  that  development  depends  not 
only  on  a  change  hi  the  surface  of  the  egg,  but  on  other 
changes  also.  Hence  his  most  successful  methods  are 
those  in  which  two  agents  are  applied  successively 
to  the  egg  —  one  affects  primarily  the  surface,  the  other 
the  interior  of  the  egg.  If,  for  example,  the  eggs  are 
placed  in  a  solution  of  a  fatty  acid,  the  membrane  is 
produced.  The  egg  is  then  removed  to  pure  sea 
water  from  which  oxygen  has  been  driven  out  and  left 
there  for  three  hours.  After  its  return  to  sea  water  it 
will  produce  a  normal  embryo. 

If,  instead  of  jnitting  the  egg  into  water  without 
oxygen,  a  hypertonic  solution  of  salts  is  used  (50  cc. 


192  HEREDITY  AND  SEX 

of  sea  water  plus  8  cc.  of  2^2  NaCl),  the  development 
may  be  carried  through. 

Loeb  concludes  that  the  oxidations  set  up  in  the  egg 
by  a  change  in  its  outer  surface  affect  the  egg  itself 
injuriously;  and  unless  they  are  removed  or  the 
effects  are  counterbalanced  by  some  other  change 
(as  when  a  hypertonic  solution  is  used)  the  egg  goes 
to  pieces.  Hence  he  believes  that  the  sperm  has  a 
double  r61e  in  fertilization.  First  it  changes  the  surface 
layer  and  increases  in  consequence  the  oxidations 
in  the  egg ;  second,  the  sperm  brings  into  the  egg  some 
substance  that  counteracts  poison  produced  by  the 
oxidation  itself. 

This  is  what  fertilization  accomplishes  from  a 
physiological  point  of  view.  In  addition,  we  have 
seen  that  fertilization  brings  into  the  egg  certain  ma- 
terials whose  presence  affects  the  characters  of  the 
individuals  that  develop  from  it.  This  is  what  fertili- 
zation does  from  the  point  of  view  of  the  student  of 
heredity. 

Let  us  turn  for  a  moment,  in  conclusion,  to  the 
question  of  sex  of  animals  that  come  from  artificially 
parthenogenetic  eggs. 

In  natural  parthenogenesis  such  eggs  may  de- 
velop into  males,  sexual  females,  or  parthenogenetic 
females. 

But  in  artificial  parthenogenesis  the  egg  has  already 
undergone  reduction  in  its  chromosomes  and  is  repre- 
sented by  half  of  the  female  formula  as  far  as  the 
chromosomes  are  concerned.  The  half  formula  will 
be  XABC  for  the  type  with  homozygous  female. 
Since  the  egg  has  one  X  it  may  be  expected  to  become 


ARTIFICIAL  PARTHENOGENESIS  193 

a  male,  but  if  sex  is  a  relation  of  X  to  ABC,  one  cannot 
be  certain  that  it  might  not  be  a  female. 

In  cases  where  the  female  is  heterozygous  for  the 
sex  factor,  as  in  birds  and  some  sea  urchins,  the  formula 
for  the  female  would  be  XABCD  —  YABCD  and  for 
the  male  YABCD  —  YABCD.  There  would  be  two 
types  of  eggs,  XABCD  and  YABCD.  The  former 
might  be  expected  to  produce  a  female,  the  latter  prob- 
ably a  male  if  such  eggs  were  incited  artificially  to 
develop. 

Concerning  the  sex  of  the  embryos  so  far  produced 
by  artificial  parthenogenesis,  we  know  of  only  two 
cases.  These  two  cases  are  Delages'  result  for  the 
sea  urchin,  in  which  he  got  one  male,  and  Loeb's  and 
Bancroft's  case  for  the  frog,  in  which  they  believe  that 
the  two  young  obtained  were  males. 

What  to  expect  on  theoretical  grounds  is  uncertain. 
We  have  only  two  facts  that  bear  on  the  question. 
In  the  parthenogenetic  eggs  of  the  aphid,  with  the  for- 
mula XABC  ABC  we  get  a  male.  In  the  case  of  the 
bee  the  formula  is  XABC,  which  also  gives  a  male.  All 
else  is  hypothetical  and  premature,  but  if  these  two 
formulae  are  correct,  it  appears  that  one  X  gives  a 
male  and  that  maleness  is  not  due  to  a  quantitative 
relation  between  X  and  one  or  two  sets  of  the  other 
chromosomes.  It  is  the  quantity  of  something  in  X, 
not  the  relation  of  this  to  the  rest  of  the  chromosomes. 


CHAPTER  VII 

FERTILITY 

DARWIN'S  splendid  work  on  cross-  and  self-fertiliza- 
tion, his  study  of  the  mechanism  of  cross-fertilization  in 
orchids,  and  his  work  on  the  different  forms  of  flowers 
of  plants  of  the  same  species,  mark  the  beginning  of 
the  modern  study  of  the  problem  of  fertility  and 
sterility.  Darwin  carried  out  studies  on  the  effects 
of  cross-fertilization  in  comparison  with  self-fertilization 
and  reached  the  conclusion  that  the  offspring  resulting 
from  cross-fertilization  are  more  vigorous  than  the 
offspring  from  self-fertilization.  No  one  can  read  his 
books  dealing  with  these  questions  without  being 
impressed  by  the  keenness  of  his  analysis  and  the 
open-minded  and  candid  spirit  with  which  the  prob- 
lems were  handled.  Since  Darwin's  time  we  have  not 
advanced  very  far  beyond  the  stage  to  which  Darwin 
carried  these  questions.  We  have  more  extensive 
experiments  and  some  more  definite  ways  of  stating 
the  results,  but  Darwin's  work  still  stands  as  the  most 
important  contribution  that  has  been  made  to  this 
subject. 

The  credit  of  the  second  advance  belongs  to  Weis- 
mann.  His  speculations  concerning  the  effects  of 
mixing  of  the  germ-plasms  of  the  two  individuals, 
that  combine  at  the  time  of  fertilization,  not  only 
aroused  renewed  interest  in  the  nature  of  the  process 
of  sexual  reproduction,  but  brought  to  light  also  the 

194 


FERTILITY  195 

effects  of  recombination  of  the  different  sorts  of  qualities 
contained  in  the  parental  strains.  His  attack  on  the 
hypothesis  of  rejuvenation  that  was  so  generally  held 
at  that  time  did  very  great  service  in  exposing  the 
mystical  nature  of  such  an  imagined  effect  of  cross- 
fertilization.  In  particular,  Weismann's  endeavor  to 
connect  the  theory  of  recombination  with  the  facts 
of  maturation  of  the  egg  and  sperm  has  opened  our 
eyes  to  possibilities  that  had  never  been  realized  before. 
His  work  has  led  directly  to  the  third  advance  that 
has  been  made  hi  very  recent  years,  when  the  results 
of  Mendelian  segregation  have  been  applied  directly 
to  the  study  of  fertility  and  sterility. 

As  I  have  said,  Darwin's  work  showed  that  cross- 
fertilization  is  generally  beneficial.  The  converse 
proposition  has  long  been  held  that  continued  inbreed- 
ing leads  to  degeneration  and  to  sterility.  This  opinion 
rests  largely  on  the  statements  of  breeders  of  domesti- 
cated animals  and  plants,  but  there  is  also  a  small 
amount  of  accurate  data  that  seems  to  support  this 
view.  I  propose  first  to  examine  this  question,  and 
then  consider  what  cross-fertilization  is  supposed  to  do, 
in  the  light  of  the  most  recent  work. 

Weismann  inbred  white  mice  for  29  generations, 
and  Ritzema-Bos  bred  rats  for  30  generations.  In 
each  case  the  number  of  young  per  litter  decreased 
in  successive  generations,  more  individuals  were  sterile 
and  many  individuals  became  weakened.  This  evi- 
dence falls  in  line  with  the  general  opinion  of  breeders. 

On  the  other  hand,  we  have  Castle's  evidence  on 
inbreeding  the  fruit  fly  through  59  generations.  He 
found  some  evidence  of  the  occurrence  of  sterile  pairs 


196  HEREDITY  AND   SEX 

(mainly  females),  but  we  must  be  careful  to  distinguish 
between  the  appearance  of  sterile  individuals  in  these 
cultures  and  the  lessened  fertility  that  may  be  shown 
by  the  stock  in  general.  The  recent  work  of  Hyde  on 
these  same  flies  has  shown  that  the  appearance  of 
sterile  individuals  may  be  an  entirely  different  question 
from  that  of  a  decrease  in  general  fertility.  The 
latter  again  may  be  due  to  a  number  of  quite  different 
conditions.  Castle  and  his  co-workers  found  that  the 
sterile  individuals  could  be  eliminated  if  in  each  genera- 
tion the  offspring  were  selected  from  pairs  that  had  not 
produced  sterile  individuals.  Hyde  has  found,  in 
fact,  that  one  kind  at  least  of  sterile  females  owe  their 
sterility  to  a  definitely  inherited  factor  that  can  be 
eliminated  as  can  any  other  Mendelian  recessive 
trait.  Moenkhaus,  who  has  also  extensively  studied  the 
problem  of  inbreeding  in  these  flies  has  likewise  found 
that  his  strains  could  be  maintained  at  their  normal 
rate  of  propagation  by  selecting  from  the  more  fertile 
pairs. 

If  we  eliminate  from  the  discussion  the  occurrence 
of  sterile  individuals,  the  question  still  remains  whether 
the  output  of  the  fertile  pairs  decreases  if  inbreeding 
is  carried  on  through  successive  generations.  There 
is  some  substantial  evidence  to  show  that  this  really 
takes  place,  as  the  following  figures  taken  from  Hyde's 
results  show. 

FI         F2        Fs        FI        F5        FQ  FU 

368      209      191       184       65       119  156 

At  the  end  of  thirteen  generations  the  fertility 
of  the  stock  was  reduced  by  half,  as  determined  in  this 


FERTILITY  197 

case  by  the  average  number  of  flies  per  pair  that 
hatch.  But  this  is  not  a  measure  of  the  number  of 
eggs  laid  or  of  those  that  are  fertilized. 

Whether  inbreeding  where  separate  sexes  exist  is  sim- 
ilar to  self-fertilization  in  hermaphroditic  forms  is  not 
known.  Darwin  gives  results  of  self-fertilization  in  Ipo- 
mcea  pur  pur  ea  for  ten  generations.  The  effects  vary  so 
much  in  successive  generations  that  it  is  not  possible 
to  state  whether  or  not  the  plant  has  become  less 
fertile.  His  evidence  shows,  however,  that  the  cross- 
fertilized  plants  in  each  of  the  same  ten  generations 
are  more  vigorous  than  the  self-fertilized  plants,  but 
this  does  not  prove  that  the  latter  deteriorated. 

The  problem  has  been  studied  in  other  ways.  Some 
animals  and  plants  propagate  extensively  by  partheno- 
genesis ;  others  by  means  of  simple  division. 

Whitney  and  A.  F.  Shull  kept  parthenogenetic  strains 
of  Hydatina  senta  for  many  generations.  Whitney 
carried  a  strain  of  this  sort  through  500  generations. 
Towards  the  end  the  individuals  became  weak,  the 
reproductive  power  was  greatly  diminished,  and  finally 
the  strain  died  out.  No  attempt  was  made  to  breed 
from  the  more  fertile  individuals,  although  to  some 
extent  this  probably  occurred  at  times.  If  we  admit 
that  weakened  individuals  appear  sometimes  in  these 
lines  and  their  weakness  is  inherited,  then  each  time 
such  an  individual  happened  to  be  picked  out  a  step 
downward  would  be  taken ;  when  the  more  fertile 
individuals  chanced  to  be  selected,  the  strain  would  be 
temporarily  held  at  that  level.  But  on  the  whole 
the  process  would  be  downwards  if  such  downward 
changes  are  more  likely  to  occur  than  upward  ones. 


198  HEREDITY  AND   SEX 

This  is  an  assumption,  but  perhaps  not  an  unreasonable 
one.  Let  me  illustrate  why  I  think  it  is  not  unreason- 
able. If  the  highest  possible  point  of  productivity 
is  a  complex  condition  due  to  a  large  number  of  things 
that  must  be  present,  then  any  change  is  more  likely 
to  be  downward,  since  at  the  beginning  the  high-water 
mark  had  been  reached.  In  time  casual  selection  would 
be  likely  to  pick  out  a  poor  combination  —  if  this  hap- 
pened once  the  likelihood  of  return  would  be  small. 

As  we  have  seen  (Chapter  I)  Maupas  found  in  a 
number  of  protozoa  that  if  he  picked  out  an  individual 
(after  each  two  divisions)  to  become  the  progenitor 
of  the  next  generation,  the  rate  of  division  after  a 
time  slowed  down.  The  individuals  became  weaker 
and  finally  the  race  died  out.  Calkins  repeated  the 
experiments  with  paramcecium  on  a  larger  scale  and 
obtained  similar  results.  The  question  arose  whether 
the  results  were  not  due  to  the  hay  infusion  lacking 
certain  chemical  substances  that  in  time  produced  an 
injurious  effect.  Calkins  tested  this  by  transferring 
his  weakened  strains  to  different  culture  media.  The 
result  was  that  the  race  was  restored  to  more  than 
its  original  vigor.  But  very  soon  degeneration  again 
set  in.  A  new  medium  again  restored  vigor  to  some 
degree,  but  only  for  a  short  time,  and  finally  the 
oldest  culture  died  out  in  the  742d  generation.  It 
was  evident,  therefore,  that  if  the  slackened  rate  of 
division  and  other  evidences  of  degeneration  were  in 
part  due  to  the  medium,  yet  some  of  the  effects 
produced  were  permanent  and  could  not  be  effaced  by 
a  return  to  a  more  normal  medium.  Then  came 
Woodruff's  experiments.  He  kept  his  paramoecia  on 


FERTILITY  199 

a  mixed  diet  —  on  the  kind  of  materials  that  it  would 
be  likely  to  meet  with  in  nature,  alternating  with  hay 
and  other  infusions.  He  found  no  degeneration,  and 
at  his  last  report  his  still  vigorous  strain  was  in  the 
3000th  generation. 

Recently  Woodruff  has  found  in  his  long-lived  race 
that  under  proper  conditions  individuals  will  conjugate. 
Woodruff  and  Erdmann  found  in  this  race,  although 
not  allowed  to  conjugate,  that  periodically  the  macro- 
nucleus  breaks  down  and  several  micromere  divisions 
take  place.  Finally  a  new  nuclear  apparatus  of  micro- 
nuclear  origin  is  reconstructed.  The  process  is  com- 
parable to  the  nuclear  changes  prior  to  conjugation 
except  that  the  last  micronu clear  division  is  omitted. 
This  periodic  change  is  not  peculiar  to  this  race  of 
Paramcecium,  but  appears  in  other  races  that  regularly 
conjugate. 

Let  us  turn  now  to  the  other  side  of  the  question 
and  see  what  results  cross-fertilization  has  given. 

Hyde  has  found  that  if  two  strains  of  flies  with  low 
fertility  are  crossed,  there  is  a  sudden  increase  in  the 
output,  as  seen  in  the  diagram  (Fig.  96).  The  facts 
show  clearly  an  improvement.  More  eggs  of  each 
strain  are  fertilized  by  sperm  from  the  other  strain 
than  when  the  eggs  are  fertilized  by  sperm  from  the 
same  strain.1  In  this  case  the  results  are  not  due 
to  a  more  fertile  individual  being  produced  (although 
this  may  be  true)  but  to  foreign  sperm,  acting  better 
than  the  strain's  own  sperm.  The  evidence,  as  such, 

1  The  upper  line  Fi-Fu  gives  the  average  output  of  flies  per  pair. 
Below  this  line  the  percentages  mean  the  number  of  isolated  eggs 
that  hatched. 


200 


HEREDITY  AND   SEX 


does  not  show  whether  this  is  due  to  each  strain  having 
degenerated  in  certain  directions,  or  to  some  other 
kind  of  a  change  in  the  heredity  complex. 

The  egg  counts  show  that  in  the  inbred  stock  many 
of  the  eggs  are  not  fertilized,  or  if  fertilized  (32%) 
they  still  fail  to  develop.  This  means  a  decrease 
in  fertility  in  the  sense  in  which  that  word  is  here 
-used.  The  offspring  that  arise  from  the  cross-fer- 
tilization of  these  strains  are  more  vigorous  than  their 
parents,  if  their  increased  fertility  be  taken  as  the 
measure  of  their  vigor.  The  latter  result  is  not  shown 
in  the  table,  for  here  52%  and  58%  are  the  percent- 
ages of  fertile  eggs  produced  when  the  two  strains  are 
crossed. 

-History  of  Irtfrred  Sfock. 

Fl       2       3       45       67       8       9      10     11      \Z    Fl3 

368     209    <9f     184     65     H9     ------     f£fc 

Cro55  of 
1rwtuate9  fry  Truncate  d 


52% 


58% 


FIG.  96.  —  The  horizontal  line  Fi-Fi3  gives  the  average  number  of  flies 
per  pair  that  emerged  from  inbred  stock,  decreasing  from  368  to  156  per  pair. 
B-elow  is  shown  the  results  of  a  cross  between  a  race  of  Truncates  (short 
wings)  and  Fis.  The  percentages  here  give  the  number  of  eggs  that  hatched 
in  each  case. 

Darwin  found  that  cross-fertilization  was  bene- 
ficial in  57  species  of  plants  that  he  studied.  In  the 


FERTILITY 


201 


case  of  primula,  which  is  dimorphic,  he  found  not  only 
that  self-fertilization  gave  less  vigorous  plants,  but 
that  when  pollen  from  a  long-styled  flower  of  one  plant 
fertilizes  the  pistil  of  another  long-styled  plant  the 
vigor  of  the  offspring  is  less  than  when  the  same  kind 
of  pollen  is  used  to  fertilize  the  pistil  of  a  short-styled 
flower.  The  next  table  gives  the  detailed  results. 


NATUBE  OF  UNION 

NUMBER  OF 
FLOWERS 
FERTILIZED 

NUMBER 
OF  SEED 
CAPSULES 

MAXIMUM 
OF  SEEDS  IN 
ANY  ONE 
CAPSULE 

MINIMUM 
OF  SEEDS  IN 
ANY  ONE 
CAPSULE 

AVERAGE 
No.  OP 
SEEDS  PER 
CAPSULE 

Long-styled  form  by 

pollen      of      short- 
styled  form: 

10 

6 

62 

34 

46.5 

Legitimate  union. 

Long-styled  form  by 

own-form  pollen: 

20 

4 

49 

2 

27.7 

Illegitimate  union. 

Short-styled  form  by 

pollen      of      long- 
styled  form  : 

10 

8 

61 

37 

47.7 

Legitimate  union. 

Short-styled  form  by 

own-form  pollen  : 

17 

3 

19 

9 

12.1 

Illegitimate  union. 

The    two    legitimate 
unions  together. 

20 

14 

62 

34 

47.1 

The  two  illegitimate 
unions  together. 

37 

7 

49 

2 

21.0 

We  know  now  that  these  two  types  of  plants  —  long- 
styled  and  short-styled  —  differ  from  each  other  by 
a  single  Mendelian  factor.  We  may  therefore  state 


202 


HEREDITY  AND  SEX 


Darwin's  result  in  more  general  terms.  The  hetero- 
zygous plant  is  more  vigorous  than  the  homozygous 
plant.  Moreover,  in  this  case  it  is  not  the  presence 
of  the  dominant  factors  that  makes  greater  vigor  (for 
the  short-styled  plant  containing  both  dominants  is 
less  vigorous  than  the  heterozygous),  but  the  presence 
of  two  different  factors  that  gives  the  result. 


FIG.  97.  —  At  left  of  figures  there  are  two  strains  of  pure  bred  corn  and 
at  right  the  hybrids  produced  by  crossing  those  two  pure  strains.  (After 
East.) 

The  most  thoroughly  worked  out  case  of  the  effects 
of  inbreeding  and  cross-breeding  is  that  of  Indian  corn. 
In  recent  years  East  and  G.  H.  Shull  have  studied  on 
a  very  large  scale  and  with  extreme  care  the  problem 
in  this  plant.  Their  results  are  entirely  in  accord  on 
all  essential  points,  and  agree  with  those  of  Collins, 
who  has  also  worked  with  corn. 

East  and  Shull  find  that  when  two  strains  of  corn 


FERTILITY 


203 


(that  have  been  to  a  large  extent  made  pure)  are  crossed, 
the  offspring  is  more  vigorous  than  either  parent  (Fig. 


FIG.  98.  —  At  left  an  ear  of  Learning  Dent  corn,  and  another  at  right 
after  four  years  of  inbreeding.  The  hybrid  between  the  two  is  shown  in  the 
middle  ear.  (After  East.) 

97).  This  is  clearly  shown  in  the  accompanying  pic- 
tures. Not  only  is  the  hybrid  plant  taller  and  stronger, 
but  in  consequence  of  this,  no  doubt,  the  yield  of  corn 


204  HEREDITY  AND   SEX 

per  bushel  is  much  increased,  as  shown  in  the  next 
figure  (Fig.  98). 

When  the  vigorous  FI  corn  is  self -fertilized,  it  produces 
a  very  mixed  progeny,  more  variable  than  itself.  Some 
of  the  F2  offspring  are  like  the  original  grandparental 
strains,  some  like  the  corn  of  first  generation,  and 
others  are  intermediate  (Fig.  99). 


FIG.  99.  —  No.  9  and  No.  12,  two  inbred  strains  of  Learning  Dent  corn 
compared  with  Fl  and  F2  (to  right).  (After  East.) 

It  will  not  be  possible  for  us  to  go  into  an  analysis 
of  this  case,  but  Shull  and  East  have  shown  that  the 
results  are  in  full  harmony  with  Mendelian  principles 
of  segregation.  The  vigor  of  the  FI  corn  is  explained 
on  the  basis  that  it  is  a  hybrid  product.  To  the  extent 
to  which  the  two  parent  strains  differ  from  each  other, 
so  much  the  greater  will  be  the  vigor  of  the  offspring. 

This  seems  an  extraordinary  conclusion,  yet  when 
tested  it  bears  the  analysis  extremely  well. 

Shull  and  apparently  East  also  incline  to  adopt  the 


FERTILITY  205 

view  that  hybridity  or  heterozygosity  itself  is  the  basis 
for  the  observed  vigor ;  but  they  admit  that  another 
interpretation  is  also  possible.  For  instance,  each  of 
the  original  strains  may  have  been  deficient  in  some  of 
the  factors  that  go  to  make  vigor.  Together  they  give 
a  more  vigorous  individual  than  themselves. 

Whitney  ran  one  line  of  hydatina  through  384  par- 
thenogenetic  generations,  when  it  died  (Line  A).  An- 
other line  was  carried  through  503  generations,  and  at 
the  last  report  was  in  a  very  weakened  condition  (Line 
B).  When  the  former  line  was  becoming  extinct,  he 
tried  inbreeding.  From  the  fertilized  eggs  he  ob- 
tained a  new  parthenogenetic  female.  It  showed 
scarcely  any  improvement.  The  other  line  gave  similar 
results.  In  one  case  he  again  inbred  for  a  second  time. 
He  found  that  the  rates  of  reproduction  of  lines  A  and 
B  were  scarcely,  if  at  all,  improved. 

Whitney  then  crossed  lines  A  and  B.  At  once  an 
improvement  was  observed.  The  rate  of  reproduction 
(vigor)  was  as  great  as  that  in  a  control  line  (reared 
under  the  same  conditions)  that  had  not  deteriorated. 

The  experiments  of  A.  F.  Shull  on  hydatina  were 
somewhat  different.  He  began  with  the  twelfth  gen- 
eration from  a  sexual  egg.  The  line  was  supposedly 
not  in  a  weakened  condition.  He  inbred  the  line  and 
obtained  from  the  fertilized  egg  a  new  parthenogenetic 
series.  After  a  few  generations  he  inbred  again.  The 
results  are  shown  in  the  next  table.  It  is  clear  that 
there  has  been  a  steady  decline  despite  sexual  repro- 
duction, measured  by  four  of  the  five  standards  that 
Shull  applied,  namely,  size  of  family  of  parthenogenetic 
females,  and  of  sexual  females,  number  of  eggs  per  day, 


206 


HEREDITY  AND   SEX 


SHOWING  DECREASE  OF  VIGOR,  AS  MEASURED  BY  VARIOUS  CHAR- 
ACTERS, IN  Six  SUCCESSIVELY  INBRED  PARTHENOGENETIC  LINES 
OF  Hydatina  senta 


m 

CHARACTER  TO  BE  MEASURED 

NUMBER  OF  PARTHENOGENETIC 
LINE 

1 

2 

3 

4 

5 

6 

I. 

Size  of  family  of  parthenogenetic  female  .     . 
Size  of  family  of  fertilized  sexual  female   .     . 
Number  of  eggs  laid  per  day    

48.4 
16.7 
11.0 
2.27 

1/11 

42.5 
12.8 
11.4 
1.66 

1/3 

46.8 
12.8 
10.3 
2.25 

2/4 

42.5 
11.5 
10.0 
1.93 

3/16 

31.0 

6.3 
9.2 
2.25 

0/4 

22.6 
7.3 
7.5 
2.12 

5/8 

Number  of  days  required  to  reach  maturity 
Proportion  of  cases  in  which  first  daughter 
did  not  become  parent    

Same  in  percentages     

14.2 

25.0 

41.6 

II. 

Size  of  family  of  parthenogenetic  female  .     . 
Size  of  family  of  fertilized  sexual  female   .     . 
Number  of  eggs  laid  per  day 

48.4 
16.7 
11.0 
2.27 

1/11 

30.8 
13.7 
11.6 
1.55 

4/9 

41.0 
13.5 
7.9 
2.57 

2/7 

37.0 
15.2 

7.7 
2.20 

2/10 

33.8 
10.1 
9.6 
1.90 

8/20 

24.8 
7.6 
8.6 
2.00 

7/16 

Number  of  days  required  to  reach  maturity 
Proportion  of  cases  in  which  first  daughter 
did  not  become  parent    

Same  in  percentages     

25.0 

23.5 

41.6 

number  of  times  the  first  daughter  was  too  weak  to 
become  the  mother  of  a  new  line.  It  is  clear  that 
inbreeding  did  not  lead  to  an  increase  in  vigor. 

In  paramcecium  there  is  also  some  new  evidence. 
Calkins  in  1904  brought  about  the  conjugation  of  two  in- 
dividuals of  a  weak  race  in  the  354th  generation.  From 
one  of  the  conjugants  a  new  line  was  obtained  that 
went  through  another  cycle  of  at  least  376  generations 
in  culture,  while  during  the  same  time  and  under  sim- 
ilar conditions  the  weakened  race  from  which  the  con- 
jugants were  derived  underwent  only  277  generations. 

Jennings  has  recently  reported  an  experiment  in 
which  some  paramcecia,  intentionally  weakened  by 
breeding  in  a  small  amount  of  culture  fluid,  were 


FERTILITY  207 

allowed  to  conjugate.  Most  of  the  lines  that  descended 
from  several  pairs  showed  no  improvement  but  soon 
died  out.  In  only  one  case  was  an  individual  produced 
that  was  benefited  by  the  process. 

Jennings'  results  are,  however,  peculiar  in  one  very 
important  respect.  He  did  not  use  a  race  that  had  run 
down  as  a  result  of  a  long  succession  of  generations,  but 
a  race  that  he  had  weakened  by  keeping  under  poor 
conditions.  We  do  not  know  that  the  result  in  this 
case  is  the  same  as  that  in  senile  races  or  inbred  races 
of  other  workers.  It  is  not  certain  that  the  hereditary 
complex  was  affected  in  the  way  in  which  that  complex 
is  changed  by  inbreeding.  He  may  have  injured  some 
other  part  of  the  mechanism. 

Jennings  interprets  conjugation  in  paramoecium  to 
mean  that  a  recombination  of  the  hereditary  factors 
takes  place.  Some  of  these  combinations  may  be  more 
favorable  for  a  given  environment  than  are  others. 
Since  these  will  produce  more  offspring,  they  will  soon 
become  the  predominant  race. 

The  next  diagram  (Fig.  100)  will  serve  to  recall  the 
principal  facts  in  regard  to  conjugation  in  paramce- 
cium.  Two  individuals  are  represented  by  black  and 
white  circles.  At  the  time  of  conjugation  the  small 
or  micronucleus  in  each  divides  (B),  each  then  divides 
again  (C).  Four  nuclei  are  produced.  One  of  these 
micronuclei,  the  one  that  lies  nearest  the  fusion  point, 
divides  once  more,  and  one  of  the  halves  passes  into  the 
other  individual  and  fuses  there  with  another  nucleus. 
The  process  is  mutual.  Separation  of  the  two  indi- 
viduals then  takes  place  and  two  ex-con jugants  are 
formed.  Each  has  a  new  double  nucleus.  This  nu- 


208  HEREDITY  AND  SEX 

cleus  divides  (G)  and  each  daughter  nucleus  divides 
again  (H),  so  that  each  ex-con jugant  has  four  nuclei. 


FIG.  100.  —  Diagram  to  show  the  history  of  the  micronuclei  of  two 
Paramoecia  during  (A-F)  and  after  (F-J)  conjugation.  Compare  this  dia- 
gram with  Fig.  2. 

Another  division  gives  eight  nuclei  in  each.  The  para- 
mcecium  itself  next  divides — each  half  gets  four  nuclei. 
A  second  division  takes  place,  and  each  gets  two  of 
the  nuclei.  Four  new  individuals  result.  In  each  of 


FERTILITY  209 

these  individuals  one  of  the  nuclei  remains  small  and 
becomes  the  new  micronucleus,  the  other  enlarges  to 
form  the  new  macronucleus.  Thus  from  each  ex- 
con  jugant  four  new  paramoecia  are  produced,  which 
now  proceed  to  divide  in  the  ordinary  way,  i.e.  the 
micronucleus  and  the  macronucleus  elongate  and  divide 
at  each  division  of  the  animal. 

It  is  customary  to  regard  some  phase  in  this  process 
as  involving  a  reduction  division  in  the  sense  that  a 
separation  of  the  paired  factors  takes  place.  If  this 
occurs  prior  to  interchange  of  micronuclei  (E),  then  each 
ex-con  jugant  corresponds  to  an  egg  after  fertilization. 
It  is  conceivable,  however,  that  segregation  might  oc- 
cur in  the  two  divisions  that  follow  conjugation,  which 
would  give  a  different  interpretation  of  the  process 
than  the  one  followed  here. 

On  the  first  of  these  two  hypotheses  two  new  strains 
result  after  conjugation.  Each  is  a  recombination  of 
factors  contained  in  the  two  parents.  If  the  two  par- 
ents were  alike,  i.e.  homozygous,  in  many  factors,  and 
different,  i.e.  heterozygous,  in  a  few,  the  two  individuals 
would  be  more  alike  than  were  the  original  races  from 
which  they  came.  This  is,  in  fact,  what  Jennings  has 
shown  to  be  the  case,  at  least  he  has  shown  that  on 
the  average  the  ex-con  jugant  s  are  more  like  each  other 
than  were  the  original  strains. 

Calkins  has  obtained  some  new  and  important  facts 
concerning  the  likeness  and  unlikeness  of  the  new 
strains  that  result  from  conjugation.  He  has  used 
wild,  i.e.  not  weakened,  individuals,  and  has  followed 
the  history  of  the  four  lines  resulting  from  the  first 
four  individuals  produced  by  each  ex-con  jugant.  The 


210 


HEREDITY  AND   SEX 


history  of  six  such  ex-con jugants  is  shown  in  the  next 
diagram  (Fig.  101).  The  four  lines,  "quadrants," 
(1,  2,  3,  4)  that  are  descended  from  each  of  six  ex- 
conjugants  (viz.  G,  #,  L,  M,  Q,  B)  are  shown.  At 
intervals  large  numbers  of  the  populations  were  put 
under  conditions  favorable  to  conjugation  and  the 


in/  »)<jc    €&  cen/uynn£i 


Mr  a.  flee.  Z)  c 


~F 


"•'& 


N<, 


/< 


v: 


1iO 

-&€- 


FIG.  101.  —  History  of  six  (G,  H,  L,  M,  Q,  B)  ex-conjugants.  In  each 
the  descendants  of  the  first  four  individuals  (after  conjugation)  is  shown; 
the  numbers  indicate  the  pairs  of  conjugants  counted  when  the  test  was 
made.  -X"  indicates  deaths;  O  indicates  that  no  conjugation  took  place. 
(After  Calkins.) 

number  of  conjugating  pairs  counted.  The  results 
are  shown  in  the  diagram.  The  circles  indicate  no 
conjugations ;  X  indicates  the  death  of  the  strain. 
In  the  G  and  in  the  M  series  many  conjugations  took 
place.  In  other  series  conjugation  did  not  take  place 
until  much  later.  Striking  differences  appear  in  the 
different  quadrants  although  they  were  kept  under 
similar  conditions. 


FERTILITY  211 

But  even  amongst  the  four  lines  descended  from  the 
same  ex-con jugant  marked  differences  exist.  These 
differences  cannot  be  attributed  to  constitutional  dif- 
ferences unless  a  segregation  of  factors  takes  place 
after  conjugation  or  unless  it  can  be  shown  that  these 
differences  are  not  significant.  In  the  light  of  these 
conflicting  results  on  paramoecium  it  may  seem  unsafe 
to  draw  any  far-reaching  conclusions  concerning  the 
nature  of  sexual  reproduction  in  general  from  the  evi- 
dence derived  from  these  forms.  In  the  higher  animals, 
however,  the  evidence  that  segregation  takes  place 
prior  to  fertilization  and  that  recombinations  result 
can  scarcely  be  doubted. 

THEORIES    OF    FERTILITY 

Let  us  now  try  to  sum  up  the  evidence  in  regard  to 
the  influence  of  cross-fertilization.  This  can  best  be 
done  by  considering  the  three  most  important  hypoth- 
eses that  have  been  brought  forward  to  explain  how 
crossing  gives  greater  vigor. 

Shull  and  East  explain  the  vigor  of  the  hybrid  by 
the  assumption  that  it  contains  a  greater  number  of  dif- 
ferent factors  in  its  make-up  than  either  of  its  parents. 
They  support  the  view  by  an  appeal  to  the  next  (F2) 
generation  from  such  hybrids  that  shows  a  lower 
range  of  vigor,  because,  while  a  few  individuals  of  this 
generation  will  be  as  mixed  as  the  hybrid  (Fi),  and 
therefore  like  it,  most  of  them  will  be  simpler  in  com- 
position. This  interpretation  is  also  supported  by  the 
evidence  that  when  pure  lines  (but  not  necessarily, 
however,  homozygous  lines)  are  obtained  by  self-fer- 
tilizing the  offspring  of  successive  generations  from 


212  HEREDITY  AND  SEX 

these  first  hybrids,  further  decline  does  not  take 
place. 

An  alternative  view,  that  is  also  Mendelian,  has  been 
offered  by  Bruce  and  by  Keeble  and  Pellew.  Vigor,  it 
is  maintained,  is  in  proportion  to  the  number  of  domi- 
nant factors,  and  in  proportion  to  the  number  of  these 
factors  present  whether  in  a  hybrid  or  in  a  homozygous 
(duplex)  condition. 

On  this  view  the  hybrid  is  vigorous,  not  because  it  is 
hybridous,  so  to  speak,  but  because  in  its  formation  a 
larger  number  of  dominant  factors  (than  were  pres- 
ent in  either  parent)  have  been  brought  together. 

A  third  view  is  also  compatible  with  the  evidence, 
namely,  that  there  may  exist  factors  that  are  them- 
selves directly  concerned  with  fertility.  There  is  one 
such  case  at  least  that  has  been  thoroughly  analyzed 
by  Pearl. 

Pearl  studied  for  five  years  the  problem  of  fertility 
in  two  races  of  fowls,  viz.  barred  Plymouth  rocks  and 
Cornish  Indian  games.  The  main  features  of  his 
results  are  shown  in  the  diagram  (Fig.  102).  He  finds 
that  the  winter  output  of  eggs,  which  is  correlated 
with  the  total  production,  is  connected  with  two  factors. 
One  factor,  designated  by  LI,  is  a  non-sex-linked  char- 
acter. If  it  is  present,  an  average  of  less  than  30  eggs 
is  produced  in  the  winter  season.  There  is  another 
factor,  L2,  that  is  present  in  the  barred  rocks,  but  not 
in  the  Indian  game.  If  present  alone,  the  winter  out- 
put is  again  about  30  eggs  on  an  average.  If,  how- 
ever, both  LI  and  L2  are  present,  the  winter  output 
is  more  than  30  and  may  be  as  great  as  90,  or  in  rare 
cases  100-120  eggs. 


FERTILITY  213 

The  peculiarity  about  this  discovery  is  that  the 
second  factor,  L2,  is  sex-linked,  which  means  in  this  case 
that  it  is  carried  by  the  eggs  that  will  produce  the  males 
in  the  next  generation,  and  not  by  the  eggs  that  will 
produce  the  daughters.  Hence  if  the  daughters  of  high- 
producing  hens  are  selected,  one  does  not  get  in  them 

irtheriran.ee   of  FertlliTy  in  fowl. 
Low  9      F.L, L,  Ft* — 


F.  L,  ^e  L,  ^          Loir  9 


Kt;";%         %$£ 

9     F—  L* 

o      l^  —  -tt 

(L£)  <3 

*>  i  a. 

LO»V  9 

T 


FIG.  102.  —  Illustrating  Pearl's  hypothesis.  F  =  female  factor  present 
in  half  of  the  eggs  and  determining  sex.  L\  =  factor  for  low  egg  produc- 
tion; /i,  its  allelomorph  for  zero  production  of  winter  eggs.  L*  =  factor 
for  high  winter  production;  /2,  its  allelomorph. 

the  high  productiveness  of  the  mother.  It  is  her  sons 
that  inherit  the  character,  although  they  cannot  show 
it  except  in  their  offspring. 

Aside  from  whatever  practical  interest  these  results 
may  have,  the  facts  are  important  in  showing  that  such 
a  thing  as  a  factor  for  fertility  itself  may  be  present, 
without  otherwise  being  apparent,  and  that  this  factor 


214 


HEREDITY  AND   SEX 


taken  in  connection  with  another  (or  others)  gives  high 
productivity. 

The  other  point  to  which  I  wish  to  call  attention 
relates  to  a  different  matter.  We  have  met  with  some 
cases  where  lowered  fertility  was  due  to  eggs  failing 


FIG.  103.  —  Normal  male  of  Drosophila  (on  left)  and  male  with  "rudi- 
mentary" wings  (on  right).     Note  sex  comb  (lower  left). 

to  a  greater  or  less  degree  to  be  fertilized  by  sperm  of 
the  same  strain. 

A  striking  case  of  this  kind  is  found  in  a  mutant  of 
the  fruit  fly  that  appeared  in  my  cultures.  The  mu- 
tant has  rudimentary  wings  (Fig.  103).  The  females 
are  absolutely  infertile  with  males  of  the  same  kind. 


FERTILITY  215 

If  they  are  mated  to  any  other  male  of  a  different  strain, 
they  are  fertilized.  The  males,  too,  are  capable  of  fer- 
tilizing the  eggs  of  other  strains,  in  fact,  are  quite 
fertile. 

The  factor  that  makes  the  rudimentary  winged 
fly  is  of  such  a  sort  that  it  carries  infertility  along  with 
it  —  in  the  sense  of  self-infertility.  This  result  has 
nothing  to  do  with  inbreeding,  and  the  stigma  cannot 
be  removed  by  crossing  out  and  extracting. 

A  somewhat  similar  factor,  though  less  marked,  is 
found  by  Hyde  in  certain  of  his  inbred  stock  to  which 
I  have  referred.  As  his  experiments  show,  the  infer- 
tility in  this  case  is  not  due  to  lack  of  eggs  or  sperm,  but 
to  a  sort  of  incompatibility  between  them  so  that  not 
more  than  20  per  cent  of  the  eggs  can  be  fertilized  by 
males  of  the  same  strain. 

In  the  flowering  plants  where  the  two  sexes  are  often 
combined  in  the  same  individual,  it  has  long  been  known 
that  there  are  cases  in  which  self-fertilization  will  not 
take  place.  The  pollen  of  a  flower  of  this  kind  if  placed 
on  the  stigma  of  the  same  flower  or  of  any  other  flower 
on  the  same  plant  will  not  fertilize  the  ovules.  Yet  the 
pollen  will  fertilize  other  plants  and  the  ovules  may  be 
fertilized  by  foreign  pollen. 

Correns  has  recently  studied  that  problem  and  has 
arrived  at  some  important  conclusions.  He  worked 
with  a  common  plant,  Cardamine  pratensis.  In  this 
plant  self-fertilization  is  ineffectual.  He  crossed  plant 
B  with  plant  G,  and  reared  their  offspring.  He  tested 
these  with  each  other  and  also  crossed  each  of  them  back 
to  its  parents  that  had  been  kept  alive  for  this  pur- 
pose. The  latter  experiment  is  simple  and  more  in- 


216 


HEREDITY  AND   SEX 


structive.     His   results   and   his   theory   can   best   be 
given  together. 

Correns  assumes  that  each  plant  contains  some  factor 
that  produces  a  secretion  on  the  stigma  of  the  flowers. 
This  secretion  inhibits  the  pollen  of  the  same  plant 
from  extending  its  pollen  tube.  He  found,  in  fact, 
that  the  pollen  grains  do  not  grow  when  placed  on  the 
stigma  of  the  same  plant.  All  plants  will  be  hybrid 


9 
rf 


BG 


B 


FIG.  104.  —  Illustrating  the  crossing  of  the  types  Bb  and  Gg  to  give  four 
classes :  BG,  Bg,  bG,  bg.  Each  of  these  is  then  back-crossed  either  to  B  or 
to  G  with  the  positive  (+)  or  negative  (  — )  results  indicated  in  the  diagram. 

for  these  factors,  hence  plant  B  will  produce  two  kinds 
of  germ-cells,  B  and  6.  Similarly,  plant  G  will  produce 
two  kinds  of  germ-cells,  G-g.  If  these  two  plants  are 
crossed,  four  types  will  be  produced.  When  these  are 
back-crossed  to  the  parents,  the  expectation  is  shown  in 
the  diagram  (Fig.  104).  Half  the  combination  should 
be  sterile  and  half  should  be  fertile.  This  is,  in  fact, 
what  occurs,  as  shown  in  the  same  diagram.  The 
—  signs  indicate  that  fertilization  does  not  occur,  while 
the  +  signs  indicate  successful  fertilization. 

Correns'  theory  is  also  in  accord  with  other  com- 


FERTILITY  217 

binations  that  he  made.  There  can  be  little  doubt 
that  he  has  pointed  out  the  direction  in  which  a  solu- 
tion is  to  be  found. 

There  is  a  somewhat  similar  case  in  animals.  In  one 
of  the  Ascidians,  Ciona  intestinalis,  an  hermaphrodite, 
the  sperm  will  not  fertilize  the  eggs  of  the  same  indi- 
vidual. But  the  sperm  will  fertilize  eggs  of  other 
individuals,  and  vice  versa.  Castle  first  found  out  this 
fact,  and  I  have  studied  it  on  a  large  scale.  The 
diagram  (Fig.  105)  gives  an  example  of  one  such  ex- 
periment made  recently  by  W.  S.  Adkins. 

Five  individuals  are  here  used.  The  eggs  of  one 
individual,  A,  were  placed  hi  five  dishes  (horizontal 
line) ;  likewise  those  of  B,  C,  D,  E.  The  sperm  of  A, 
designated  by  a  (vertical  lines)  was  used  to  fertilize 
the  eggs,  A,  B,  C,  D,  E ;  likewise  the  sperm  6,  c,  d,  e. 
The  self-fertilized  sets  form  the  diagonal  line  in  the 
diagram  and  show  no  fertilization.  The  other  sets 
show  various  degrees  of  success,  as  indicated  by  the 
percentage  figures.  These  results  can  best  be  under- 
stood, I  think,  by  means  of  the  following  hypoth- 
esis. The  failure  to  self-fertilize,  which  is  the  main 
problem,  would  seem  to  be  due  to  the  similarity  in  the 
hereditary  factors  carried  by  eggs  and  sperm ;  but 
in  the  sperm,  at  least,  reduction  division  has  taken 
place  prior  to  fertilization,  and  therefore  unless  each 
animal  was  homozygous  (which  from  the  nature  of  the 
case  cannot  be  assumed  possible)  the  failure  to  fertilize 
cannot  be  due  to  homozygosity.  But  both  sperm  and 
eggs  have  developed  under  the  influence  of  the  total 
or  duplex  number  of  hereditary  factors;  hence  they 
are  alike,  i.e.  their  protoplasmic  substance  has  been 


218 


HEREDITY  AND   SEX 


under  the  same  influences.     In  this  sense,  the  case  is 
like  that  of  stock  that  has  long  been  inbred,  and  has 


jSetf  and 


in  Gona. 


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FIG.  105.  —  The  oblique  line  of  letters  A°,  56,  Cc,  Z)d,  ^e,  gives  the  self- 
fertilized  sets  of  eggs;  the  rest  Ab,  Ac,  etc.,  the  cross-fertilized  sets.  A,  B, 
C,  D,  E  =  eggs ;  a,  b,  c,  d,  e,  =  sperm  of  same  individuals.  (From  unpub- 
lished work  of  W.  S.  Adkins.) 

come  to  have  nearly  the  same  hereditary  complex.  If 
this  similarity  decreases  the  chances  of  combination  be- 
tween sperm  and  eggs,  we  can  interpret  the  results.  Cor- 
rens'  results  may  come  under  the  same  interpretation. 


FERTILITY  219 

I  have  tried  to  bring  together  the  modern  evidence 
that  bears  on  the  problems  of  fertility  and  sterility. 
It  is  evident  that  there  are  many  obscure  relations  that 
need  to  be  explained.  I  fear  that,  owing  to  the  diffi- 
culty of  summarizing  this  scattered  and  diverse  ma- 
terial, I  have  failed  to  make  evident  how  much  labor 
and  thought  and  patience  has  been  expended  in  ob- 
taining these  results,  meager  though  they  may  appear. 

But  while  it  is  going  to  take  a  long  time  and  many 
heads  and  hands  to  work  out  fully  these  problems,  there 
can  be  little  doubt  that  the  modern  method  is  the  only 
one  by  which  we  can  hope  to  reach  a  safe  conclusion. 


CHAPTER  VIII 
SPECIAL  CASES  OF  SEX-INHERITANCE 

THE  mechanism  of  sex-determination  that  we  have 
examined  gives  equal  numbers  of  males  and  females. 
But  there  are  known  certain  special  cases  where  equality 
does  not  hold.  I  have  selected  six  such  cases  for 
discussion.  Each  of  these  illustrates  how  the  mechan- 
ism of  sex-determination  has  changed  to  give  a  different 
result ;  or  how,  the  mechanism  remaining  the  same,  some 
outside  condition  has  come  in  that  affects  the  sex  ratio. 

It  is  so  important  at  the  outset  to  clearly  recognize 
the  distinction  between  sex-determination  and  sex 
ratio,  that  I  shall  take  this  opportunity  to  try  to  make 
clear  the  meaning  of  this  distinction.  The  failure  to 
recognize  the  distinction  has  been  an  unfailing  source 
of  niisunderstanding  in  the  literature  of  sex.. 

(1)  A  hive  of  bees  consists  of  a  queen,  thousands  of 
workers,  and  at  certain  seasons  a  few  hundred  drones 
or  males.  The  workers  are  potentially  females,  and 
these  with  the  queen  give  an  enormous  preponderance 
of  females.  In  this  case  the  explanation  of  the  sex 
ratio  is  clear.  Most  of  the  eggs  laid  by  the  queen  are 
fertilized,  and  in  the  bee  all  fertilized  eggs  become  fe- 
males, because  as  we  have  seen  there  is  only  one  class  of 
spermatozoa  produced,  and  not  two  as  in  other  insects. 

There  is  a  parallel  and  interesting  case  in  one  of  the 
wasps  described  by  Fabre.  The  female  lays  her  eggs 

220 


SPECIAL  CASES  OF  SEX-IXHERITAXCE         221 

as  a  rule  in  the  hollow  stems  of  plants,  each  egg  in  a 
separate  compartment.  In  some  of  the  compartments 
she  stores  away  much  more  food  than  in  others.  From 
these  compartments  large  females  hatch.  From  com- 
partments where  less  food  is  stored  the  smaller  males 
are  produced.  It  may  seem  that  the  amount  of  food 
stored  up  determines  the  sex  of  the  bee.  To  test  this 
Fabre  took  out  the  excess  of  food  from  the  large 
compartments.  The  wasp  that  emerged,  although 
small  for  want  of  food,  was  in  every  case  a  female. 
Fabre  enlarged  the  smaller  compartments  and  added 
food.  The  wasp  that  came  out  was  a  male,  larger 
than  the  normal  male. 

It  is  evident  that  food  does  not  determine  the  sex, 
but  the  mother  wasp  must  fertilize  the  eggs  that  she 
lays  in  chambers  where  she  has  stored  up  more  food, 
and  not  fertilize  those  eggs  that  she  deposits  in  com- 
partments where  she  has  accumulated  less  food. 

(2)  A  curious  sex  ratio  appeared  in  one  race  of  fruit 
flies.  Some  of  the  females  persisted  hi  producing  twice 
as  many  females  as  males.  This  was  first  discovered 
by  Miss  Rawls.  In  order  to  study  what  was  taking 
place,  I  bred  one  of  these  females  that  had  red  eyes  to 
a  white-eyed  male  of  another  stock.  All  the  offspring 
had  red  eyes,  as  was  to  be  expected.  I  then  bred  these 
daughters  individually  to  white-eyed  males  again 
(Fig.  106).  Half  of  the  daughters  gave  a  normal 
ratio ;  the  other  half  gave  the  following  ratio : 

Red  Red  White  White 

$<??<? 
50  0  50  50 


222 


HEREDITY  AND   SEX 


It  is  evident  that  one  class  of  males  has  failed  to  ap- 
pear —  the  red  males.  If  we  trace  their  history  through 
these  two  generations,  we  find  that  the  single  sex  chro- 


FIG.  106.  —  Diagram  to  show  the  heredity  of  the  lethal  factor  (carried 
by  black  X).  A,  red-eyed  female,  carrying  the  factor  in  one  -X",  is  bred  to 
normal  white-eyed  male.  B,  her  red-eyed  daughter,  is  bred  again  to  a  normal 
white-eyed  male,  giving  theoretically  the  four  classes  shown  in  C,  but  one  of 
the  classes  fails  to  appear,  viz.  the  red-eyed  male  (colored  black  in  the  dia- 
gram). The  analysis  (to  right)  shows  that  this  male  has  the  fatal  X.  One 
of  his  sisters  has  it  also,  but  is  saved  by  the  other  X.  She  is  the  red-eyed 
female.  If  she  is  bred  to  a  white-eyed  male,  she  gives  the  results  shown  in 
D,  in  which  one  class  of  males  is  again  absent,  viz.  the  red-eyed  male.  In 
this  diagram  the  black  X  represents  red  eyes  and  lethal  (as  though  completely 
linked). 


SPECIAL  CASES  OF  SEX-INHERITANCE         223 

mosome  that  each  red  male  contains  is  one  of  the  two 
chromosomes  present  in  the  original  red-eyed  grand- 
mother. If  this  chromosome  contains  a  factor  which 
if  present  causes  the  death  of  the  male  that  contains  it, 
and  this  factor  is  closely  linked  to  the  red  factor, 
the  results  are  explained.  All  the  females  escape  the 
fatality,  because  all  females  contain  two  sex  chromo- 
somes. If  a  female  should  contain  the  fatal  factor, 
her  life  is  saved  by  the  other,  normal,  sex  chromosome. 
The  hypothesis  has  been  tested  in  numerous  ways  and 
has  been  verified.  We  keep  this  stock  going  by  mat- 
ing the  red  females  to  white  males.  This  gives  con- 
tinually the  2  :  1  ratio.  The  white  sisters,  on  the  other 
hand,  are  normal  and  give  normal  sex  ratios. 

(3)  Another  aberrant  result,  discovered  by  C.  B. 
Bridges,  is  shown  by  a  different  race  of  these  same  fruit 
flies.  It  will  be  recalled  that  when  an  ordinary  white- 
eyed  female  is  bred  to  a  red-eyed  male  all  the  sons  have 
white  eyes.  But  in  the  race  in  question  a  different  re- 
sult follows,  as  shown  by  the  diagram.  From  90  to 
95  per  cent  of  the  offspring  are  regular,  but  5  per  cent 
of  the  females  and  5  per  cent  of  the  males  are  uncon- 
formable,  yet  persistently  appear  in  this  stock. 

The  results  can  be  explained  if  we  suppose  that  the 
egg  contains  two  X's  and  a  Y  chromosome  and  in  con- 
sequence the  two  X's  may  pass  out  into  one  of  the 
polar  bodies,  in  which  case  the  red-eyed  males  will 
develop  if  the  egg  is  fertilized  by  a  female-producing 
sperm;  or  the  two  X-chromosomes  will  both  stay  in 
the  egg,  and  give  a  kind  of  female  with  three  sex  chro- 
mosomes. 

Here  also  numerous  tests  can  be  made.     They  verify 


224 


HEREDITY  AND   SEX 


the  expectation.  Thus  by  utilizing  sex  chromosomes 
that  carry  other  sex-linked  characters  than  white  eyes, 
it  can  be  shown  that  the  results  are  really  due  to  the 
whole  sex  chromosome  being  involved,  and  not  to 
parts  of  it.  The  result  is  of  unusual  interest  in  another 
direction ;  for  it  shows  that  the  female-producing 


FIG.  107.  —  Non-disjunction.  A  non-disjunction  female  produces  four 
types  of  eggs,  viz.,  XY  —  X  —  XX —  Y.  Such  a  female  will  give,  with  a 
normal  male,  XY,  the  classes  indicated  on  the  diagram.  Y  should  be  sub- 
stituted for  0  in  the  diagram.  The  non-disjunction  9  is  XX Y. 

sperm  will  make  a  male  if  it  enters  an  egg  from  which 
both  sex  chromosomes  have  been  removed.  It  is 
therefore  not  the  female-producing  sperm,  as  such, 
that  gives  a  female  under  normal  conditions,  but  this 
sperm  plus  the  sex  chromosome  already  present  in 
the  egg  that  gives  an  additive  result  — *a  female. 

(4)  In  the  group  of  nematode  worms  belonging  es- 
pecially to  the  genus  Rhabditis,  there  are  some  extraor- 


SPECIAL  CASES  OF  SEX-INHERITANCE        225 


dinary  perversions  of  the  sex  ratios.     The  table  gives 
the  ratios  that  Maupas  discovered.     Not  only  are  the 


Diplogaster  robustus 0. 13  male 

Rhabditis  guignardi 0.15  male 

Rhabditis  dulichura 0.7    male 

Rhabditis  caussaneli 1.4    males 

Rhabditis  elyaus 1.5    males 


to  1000  females 


Rhabditis  coronata 5.0  males 

Rhabditis  perrieri 7.0  males 

Rhabditis  marionii 7.6  males 

Rhabditis  duthiersi 20.0  males 

Rhabditis  viguieri 45.0  males 


males  extremely  rare  —  almost  reaching  a  vanishing 
point  in  certain  cases  —  but  they  have  lost  the  instinct 
to  fertilize  the  female. 

The  females,  on  the  other  hand,  have  acquired  the 
power  of  producing  sperm,  so  that  they  have  passed 
over  into  the  hermaphroditic  state.  The  behavior 
and  history  of  the  sperm  that  the  females  produce  has 
only  recently  been  made  out  by  Miss  Eva  Krtiger. 
It  is  found  that  a  spermatozoon  enters  each  egg  and 
starts  the  development,  but  takes  no  further  part  in 
the  development  (Fig.  108).  The  egg  may  be  said  to 
be  half  fertilized.  It  is  a  parthenogenetic  egg  and 
produces  a  female. 

(5)  Some  very  high  male  ratios  have  been  reported 
by  Guyer  in  cases  where  birds  of  very  different  families 
have  been  crossed  —  the  common  fowl  by  the  guinea 
hen,  individuals  of  different  genera  of  pheasants  bred 
to  each  other  and  to  fowls,  etc.  Hybrids  between 
different  genera  gave  74  $  -  -  13  9  .  Hybrids  between 
different  species  of  the  same  genus  72  $  -  -  18  9  .  In 
most  of  these  cases,  as  Guyer  points  out,  the  sex  is 


226  HEREDITY  AND  SEX 

recorded  from  the  mounted  museum  specimen  which 
has  the  male  plumage.  But  it  is  known  that  the  re- 
productive organs  of  hybrids,  extreme  as  these,  are  gen- 
erally imperfect  and  the  birds  are  sterile.  It  has  been 

Fig.  2. 


Fig.  5. 
Fig.  4. 


Fig.  12 
Fig.  7.  Fig.  8.  FiR-9.  Fig.  10.        Fig.  11.        ^ 


FIG.  108.  —  Oogenesis  and  spermatogenesis  of  Rhabditis  aberrans. 
1-5,  stages  in  oogenesis,  including  incomplete  attempt  to  form  one  polar 
body.  Eighteen  chromosomes  in  1  and  again  in  4  and  5.  In  3  the  entering 
sperm  seen  at  right.  6,  prophase  of  first  spermatocyte  with  8  double  and 
two  single  chromosomes  (sex  chromosomes).  At  the  first  division  (7)  the 
double  chromosomes  separate,  and  the  two  sex  chromosomes  divide,  giving 
ten  chromosomes  to  each  daughter  cell  (8).  At  the  next  division  the  two 
sex  chromosomes  move  to  opposite  poles,  giving  two  female-producing 
sperm  (9  and  10).  Rarely  one  of  them  may  be  left  at  the  division  plane 
and  lost,  so  that  a  male-producing  sperm  results  that  accounts  for  the  rare 
occurrence  of  males.  (After  E.  Kriiger.) 

shown  that  if  the  ovary  of  the  female  bird  is  removed 
or  deficient,  she  assumes  the  plumage  of  the  male. 
Possibly,  therefore,  some  of  these  cases  may  fall  under 
this  heading,  but  it  is  improbable  that  they  can  all  be 
explained  in  this  way.  In  the  cases  examined  by  Guyer 
himself  the  hybrids  were  dissected  and  all  four  were 
found  to  be  males. 


SPECIAL  CASES   OF  SEX-INHERITANCE         227 


Pearl  has  recently  pointed  out  that  the  sex  ratio 
in  the  Argentine  Republic  varies  somewhat  accord- 
ing to  whether  individuals  of  the  same  race,  or  of  dif- 
ferent races,  are  the  parents.  As  seen  in  the  following 
table,  the  sex  ratio  of  Italian  by  Italian  is  100.77; 

COMPARISON  OF  THE  SEX  RATIOS  OF  THE  OFFSPRING  OF  PURE  AND 
CROSS  MATINGS 


MATINGS 

SEX  RATIO 

DIFFERENCE 

P.E.  OF  DIFFERENCE 

Italian  $ 
Italian  $ 

Argentine  9 
Italian  9 

105.72  ±.46 
100.77  ±.20 

Difference 

4.95  ±.50 

9.9 

Italian  $ 
Argentine  $ 

Argentine  9 
Argentine  9 

105.72  ±.46 
103.26  ±.34 

Difference 

2.46  ±.57 

4.3 

Spanish  $ 
Spanish  $ 

Argentine  9 
Spanish  9 

106.69  ±.74 
105.55  ±.36 

Difference 

1.14  ±.82 

1.4 

Spanish  $ 
Argentine  $ 

Argentine  9 
Argentine  9 

106.69  ±.74 
103.26  ±.34 

Difference 

3.43  ±.81 

4.2 

Argentine  by  Argentine,  103.26  ;  but  Italian  by  Argen- 
tine, 105.72.  If,  as  has  so  often  been  found  to  be  the 
case,  a  hybrid  combination  gives  a  more  vigorous 
progeny,  the  higher  sex  ratio  of  the  cross-breed  may 
account  for  the  observed  differences,  since  other  data 
show  that  the  male  infant  is  less  viable  and  the  in- 
creased vigor  of  a  hybrid  combination  may  increase 
the  chance  of  survival  of  the  male. 


228    •  HEREDITY  AND   SEX 

(6)  We  come  now  to  the  most  perplexing  case  on 
record.  In  frogs  the  normal  sex  ratio  is  approximate 
equality.  Professor  Richard  Hertwig  has  found  that 
if  the  deposition  of  the  eggs  is  prevented  for  two  to 
three  days  (after  the  eggs  have  reached  the  uterus) 
the  proportion  of  males  is  enormously  increased  - 
in  the  extreme  case  all  the  offspring  may  be  males. 
By  critical  experiments  Hertwig  has  shown  that  the 
results  are  not  due  to  the  age  of  the  spermatozoa,  al- 
though in  general  he  is  inclined  to  attribute  certain 
differences  in  sex-determination  to  the  sperm  as  well 
as  to  the  eggs. 

The  evidence  obtained  by  his  pupil,  Kuschakewitsch, 
goes  clearly  to  show  that  the  high  male  sex  ratio  is 
not  due  to  a  differential  mortality  of  one  sex. 

In  the  following  table  four  experiments  (a,  6,  c,  d) 
are  summarized.  The  interval  between  each  record 

/72\ 

a)  47  9  :  32  $  0  9  :  97  $ 


b)  34  9:  472          659:772  156?:  1942        79:482 

/36\  /18\ 

c)  64  ?:  61  £         1019:1392         1159:1692 


d)  55  9: 52  $        1489:872  719:702         179:1292 

is  written  above  in  hours.  In  all  cases  an  excess  of 
males  is  found  if  the  eggs  have  been  kept  for  several 
hours  before  fertilization.  In  the  first  (a),  second  (6), 
and  fourth  (d)  cases  the  excess  of  males  is  very  great. 
Hertwig  attempts  to  bring  his  results  into  line  with 


SPECIAL  CASES   OF  SEX-INHERITANCE     .    229 

his  general  hypothesis  of  nucleo-plasm  relation.  He 
holds,  for  instance,  that  sex  may  be  determined  by  the 
relation  between  the  size  of  the  nucleus  and  the  proto- 
plasm of  the  cell.  As  the  value  of  the  evidence  has 
been  seriously  called  into  question  in  general,  and  as 
there  is  practically  no  evidence  of  any  weight  in  its  favor 
in  the  present  case,  I  shall  not  dwell  further  on  the 
question  here.  But  the  excessively  high  male  ratio  is 
evident  and  positive.  How  to  explain  it  is  difficult 
to  say.  It  is  just  possible,  I  think,  that  delay  may  have 
injured  the  egg  to  such  an  extent  that  the  sperm  may 
start  the  development  but  fail  to  fuse  with  the  egg 
nucleus.  Under  these  circumstances  there  is  the  possi- 
bility that  all  the  frogs  would  be  males. 

Miss  King  has  also  carried  out  extensive  sets  of  ex- 
periments with  the  common  toad.  She  has  studied  the 
eggs  and  the  sperm  under  many  different  conditions,  such 
as  presence  of  salt  solutions,  acids,  sugar  solutions,  cold, 
and  heat.  Her  results  are  important,  but  their  inter- 
pretation is  uncertain.  In  sugar  solutions  and  in  dry 
fertilization  the  males  decreased,  in  the  latter  from 
114.10  to  29.41  per  100  ??.  The  normal  sex  ratio  for 
the  toad  is  90  $  to  100  ?  .  Whether  the  solutions  have  in 
any  sense  affected  the  determination  of  sex,  or  acted  to 
favor  one  class  of  sperm  at  the  expense  of  the  other  re- 
mains to  be  shown,  as  Miss  King  herself  frankly  admits. 

In  the  case  of  man  there  are  extensive  statistics 
concerning  the  birth  rate.  The  accompanying  tables 
give  some  of  the  results.  There  are  in  all  parts  of  the 
world  more  males  born  than  females.  The  excessively 
high  ratios  reported  from  the  Balkans  (not  given  here) 
may  be  explained  on  psychological  grounds,  as  failure 


230 


HEREDITY  AND   SEX 


MALES 

Italy 105.8 

France 104.6 

England 103.6 

Germany 105.2 

Austria 105.8 

Hungary 105.0      to  100  females 

Switzerland 104.5 

Belgium 104.5 

Holland 105.5 

Spain 108.3 

Russia 105.4 

to  report  the  birth  of  a  boy  is  more  likely  to  lead  to  the 
imposition  of  a  fine  on  account  of  the  conscription. 

There  can  be  no  doubt,  however,  that  slightly  more 
males  than  females  are  born.  Moreover,  if  the  still- 
born infants  alone  are  recorded,  surprisingly  large  ratios 
occur,  as  shown  in  the  next  table. 

MALES 

Italy     . 131.1 

France      .......  142.2 

Germany 128.3 

Austria 132.1 

Hungary 130.0 

Switzerland 135.0      to  100  females 

Belgium 132.1 

Holland 127.7 

Sweden 135.0 

Norway 124.6 

Denmark 132.2 

And  if  abortive  births  are  also  taken  into  account,  the 
ratio  is  still  higher.  It  seems  that  the  male  embryo 
is  not  so  strong  as  the  female,  or  else  less  likely,  from 
other  causes,  to  be  born  alive. 

In  many  of  the  domesticated  animals  also,  especially 


SPECIAL  CASES  OF  SEX-INHERITANCE         231 

the  mammals,  there  is  an  excess  of  males  at  birth,  as 
the  next  table  shows. 

MALES  FEMALES 

Horse 98.31  100  (Busing) 

Cattle 107.3  100  (Wilckens) 

Sheep 97.7  100  (Darwin) 

Pig 111.8  100  (Wilckens) 

Rat 105.0  100  (Cuenot) 

Dove 105.0  100  (Cuenot) 

Hen 94.7  100  (Darwin) 

A  little  later  I  shall  bring  forward  the  evidence  that 
makes  probable  the  view  that  in  man  the  mechanism 
for  sex-determination  is  like  that  hi  other  animals, 
where  two  classes  of  sperm  are  produced,  male-  and 
female-producing.  How  then  can  we  account  in  the 
human  race  for  the  excess  of  eggs  that  are  fertilized 
by  male-producing  spermatozoa  ?  At  present  we  do  not 
know,  but  we  can  at  least  offer  certain  suggestions  that 
seem  plausible. 

In  mammals  the  fertilization  occurs  in  the  upper 
parts  of  the  oviduct.  In  order  to  reach  these  parts 
the  sperm  by  their  own  activity  must  traverse  a  dis- 
tance relatively  great  for  such  small  organisms.  If 
the  rate  of  travel  is  ever  so  slightly  different  for  the  two 
classes  of  sperm,  a  differential  sex  ratio  will  occur. 

Again,  if  from  any  cause,  such  as  disease  or  alcoholism, 
one  class  of  sperm  is  more  affected  than  the  other,  a 
disturbance  in  the  sex  ratio  would  be  expected. 

At  present  these  are  only  conjectures,  but  I  see 
no  ground  for  seizing  upon  any  disturbance  of  the 
ratio  in  order  to  formulate  far-reaching  conclusions 
in  regard  to  sex-determination  itself.  As  I  pointed 
out  in  the  beginning  of  this  chapter,  we  may  go 


232  HEREDITY  AND   SEX 

wide  of  the  mark  if  we  attempt  to  draw  conclusions 
concerning  the  determination  of  sex  itself  from  devia- 
tions such  as  these  in  the  sex  ratio,  yet  it  is  the  mistake 
that  has  been  made  over  and  over  again.  We  must 
look  to  other  methods  to  give  us  sufficient  evidence  as 
to  sex-determination.  Fortunately  we  are  now  in  a 
position  to  point  to  this  other  evidence  with  some 
assurance.  With  the  mechanism  itself  worked  out, 
we  are  in  a  better  position  to  explain  slight  variations 
or  variables  that  modify  the  combinations  in  this  way 
or  in  that. 

THE   ABANDONED    VIEW   THAT   EXTERNAL   CONDITIONS 
DETERMINE    SEX 

But  before  taking  up  the  evidence  for  sex-determina- 
tion in  man  I  must  briefly  consider  what  I  have  been 
bold  enough  to  call  the  abandoned  view  that  external 
conditions  determine  sex. 

Let  us  dismiss  at  once  many  of  the  guesses  that  have 
been  made.  Drelincourt  recorded  262  such  guesses, 
and  Geddes  and  Thomson  think  that  this  number  has 
since  been  doubled.  Naturally  we  cannot  consider 
them  all,  and  must  confine  ourselves  to  a  few  that 
seem  to  have  some  basis  in  fact  or  experiment. 

The  supposed  influence  of  food  has  been  utilized  in 
a  large  number  of  theories.  The  early  casual  evidence 
of  Landois,  of  Mrs.  Treat,  and  of  Gentry  has 
been  entirely  set  aside  by  the  careful  observations  of 
Riley,  Kellogg  and  Bell,  and  Cu6not.  In  the  latter 
cases  the  experiments  were  carried  through  two  or  even 
three  generations,  and  no  evidence  of  any  influence  of 
nourishment  was  found. 


SPECIAL  CASES  OF  SEX-INHERITANCE         233 

The  influence  of  food  in  sex-determination  in  man  has 
often  been  exploited.  It  is  an  ever  recurrring  episode 
in  the  ephemeral  literature  of  every  period.  The  most 
noted  case  is  that  of  Schenk.  In  his  first  book  he  said 
starvation  produced  more  females ;  in  his  second  book 
he  changed  his  view  and  supposed  that  starvation 
produces  more  males. 

Perhaps  the  most  fertile  source  from  which  this  view 
springs  is  found  in  some  of  the  earlier  statistical  works, 
especially  that  of  Busing.  Busing  tried  to  show  that 
more  girls  are  born  in  the  better-fed  classes  of  the  com- 
munity, in  the  poorer  classes  more  boys.  The  effective 
difference  between  these  two  classes  is  supposedly  one  of 
food  !  For  instance,  he  states  that  the  birth-rate  for 
the  Swedish  nobility  is  98  boys  to  100  girls,  while  in  the 
Swedish  clergy  the  birth-rate  is  108.6  boys  to  100  girls. 

Other  statistics  give  exactly  opposite  results.  Pun- 
nett  found  for  London  (1901)  more  girls  born  amongst 
the  poor  than  the  rich.  So  many  elements  enter  into 
these  data  that  it  is  doubtful  if  they  have  much  value 
even  in  pointing  out  causes  that  affect  the  sex  ratio,  and 
it  is  quite  certain  that  they  throw  no  light  on  the 
causes  that  determine  sex. 

In  other  mammals  where  a  sex  ratio  not  dissimilar 
to  that  in  man  exists,  extensive  experiments  on  feeding 
have  absolutely  failed  to  produce  any  influence  on 
the  ratio.  We  have,  for  instance,  Cu6not's  experi- 
ments with  rats,  and  Schultze's  experiments  with 
mice.  The  conditions  of  feeding  and  starvation  were 
much  more  extreme  in  some  cases  than  is  likely  to 
occur  ordinarily,  yet  the  sex  ratio  remained  the  same. 

Why  in  the  face  of  this  clear  evidence  do  we  find 


234  HEREDITY  AND   SEX 

zoologists,  physicians,  and  laymen  alike  perpetually 
discovering  some  new  relation  between  food  and  sex? 
It  is  hard  to  say.  Only  recently  an  Italian  zoologist, 
Russo,  put  forward  the  view  that  by  feeding  animals 
on  lecithin  more  females  were  produced.  He  claimed 
that  he  could  actually  detect  the  two  kinds  of  eggs 
in  the  ovary  —  the  female-  and  the  male-producing.  It 
has  been  shown  that  his  data  were  selected  and  not 
complete ;  that  repetition  of  his  experiments  gave  no 
confirmative  results,  and  probably  that  one  of  the  two 
kinds  of  eggs  that  he  distinguished  were  eggs  about  to 
degenerate  and  become  absorbed. 

But  the  food  theories  will  go  on  for  many  years  to 
come  —  as  long  as  credulity  lasts. 

Temperature  also  has  been  appealed  to  as  a  sex  fac- 
tor in  one  sense  or  another.  R.  Hertwig  concluded 
that  a  lower  temperature  at  the  time  of  fertilization 
gave  more  male  frogs,  but  Miss  King's  observations 
failed  to  confirm  this.  There  is  the  earlier  work  of 
Maupas  on  hydatina  and  the  more  recent  work  of 
von  Malsen  on  Dinophilus  apatris.  I  have  already 
pointed  out  that  Maupas'  results  have  not  been  con- 
firmed by  any  of  his  successors.  Even  if  they  had  been 
confirmed  they  would  only  have  shown  that  tempera- 
ture might  have  an  effect  in  bringing  parthenogenesis 
to  an  end  and  instituting  sexual  reproduction  in  its 
stead.  In  hydatina  the  sexual  female  and  the  male 
producing  individual  are  one  and  the  same.  A  more 
striking  case  could  not  be  found  to  show  that  the  en- 
vironment does  not  determine  sex  but  may  at  least 
change  one  method  of  reproduction  into  another. 

There  remain  von   Malsen's  results  for  dinophilus, 


SPECIAL  CASES  OF  SEX-INHERITANCE         235 

where  large  and  small  eggs  are  produced  by  the  same 
female  (Fig.  109).  The  female  lays  her  eggs  in  clus- 
ters, from  three  to  six  eggs,  as  a  rule,  in  each  cluster. 
The  large  eggs  produce  females ;  the  small  eggs  pro- 


FIG.  109.  —  DinophUus  gyrdciliatus.  Females  (above  and  to  left)  and 
males  (below  and  to  right).  Two  kinds  of  eggs  shown  in  middle  of  lower 
row.  (After  Shearer.) 

duce  rudimentary  males  that  fertilize  the  young  fe- 
males as  soon  as  they  hatch  and  before  they  have  left 
the  jelly  capsule. 

Von  Malsen  kept  the  mother  at  different  tempera- 
tures, with  the  results  shown  in  the  table.  The  ratio 
of  small  eggs  to  large  eggs  changes.  But  the  result 


TEMPERATURE 

No.  OF 
BROODS 

d" 

9 

SEX 
RATIO 

EGGS  PER 
BROOD 

Room  temp.  19°  C.   . 

202 

327 

813 

1:2,4 

5,6 

Cold,  13°  C.      .     .     . 

925 

973 

2975 

1:3,5 

4,2 

Heat,  26°  C.      .     .     . 

383 

507 

886 

1:  1,7 

3,6 

236  HEREDITY   AND   SEX 

obviously  may  only  mean  that  more  of  the  large  eggs 
are  likely  to  be  laid  at  one  temperature  than  at  another. 
In  fact,  temperature  seemed  to  act  so  promptly  accord- 
ing to  Von  Malsen's  observations  that  it  is  very  un- 
likely that  it  could  have  had  any  influence  in  deter- 
mining the  kind  of  egg  produced,  but  rather  the  kind 
of  egg  that  was  more  likely  to  be  laid.  We  may  dis- 
miss this  case  also,  I  believe,  as  not  showing  that  sex 
is  determined  by  temperature. 

SEX-DETERMINATION   IN   MAN 

i 

Let  us  now  proceed  to  examine  the  evidence  that 
bears  on  the  determination  of  sex  in  man.  I  shall 
draw  on  three  sources  of  evidence : 

1.  Double  embryos  and  identical  twins. 

2.  Sex-linked  inheritance  in  man. 

3.  Direct  observations  on  the  chromosomes. 

The  familiar  case  of  the  Siamese  twins 'is  an  example 
of  two  individuals  organically  united.  A  large  "series 
of  such  dual  forms  is  known  to  pathologists.  There 
are  hundreds  of  recorded  cases.  In  all  of  these  both 
individuals  are  of  the  same  sex,  i.e.  both  are  males 
or  both  are  females.  There  is  good  evidence  to  .show 
that  these  double  types  have  come  from  a  single  fer- 
tilized egg.  They  are  united  in  various  degrees  (Fig. 
110) ;  only  those  that  have  a  small  connecting  region 
are  capable  of  living.  These  cases  lead  directly  to 
the  formation  of  separate  individuals,  the  so-called 
identical  twins. 

Galton  was  one  of  the  first,  if  not  the  first,  to  recognize 
that  there  are  two  kinds  of  i  wins  —  identical  twins  and 
"ordinary  or  fraternal  twins. 


SPECIAL  CASES   OF  SEX-INHERITANCE         237 

Identical  twins  are,  as  the  name  implies,  extremely 
alike.  They  are  always  of  the  same  sex.  There 
is  every  presumption  and  some  collateral  evidence 
to  show  that  they  come  from  one  egg  after  fer- 
tilization.' On  ^Ene  other  hand,  amongst  ordinary 
twins  a  boy  and  a  girl,  or  two  boys  and  two  girls,  occur 
in  the  ratio  expected,  i.e.  on  the  basis  that  their  sex  is 


.  tit  it  it 


DIAGRAM  SHOWING  THE  INTERRELATIONS  OF  THE  VARIOUS  SORTS  OF  OlPLOPAGI  ANO 
DUPLICATE  TWINS,  ILLUSTRATIVE  OF  THE  THEORY  ADVANCED  IN  THIS  PAPER.  FURTHER  EX- 
PLANATION IN  THE  TEXT. 

FIG.  110.  —  Diagram  showing  different  types  of  union  of  double  monster 
(After  Wilder.) 

not  determined  by  a  common  external  or  internal 
cause.  Since  fraternal  twins  and  identical  twins  show 
these  relations  at  birth  and  from  the  fact  that  they 
have  been  in  both  cases  subjected  to  the  same  condi- 
tions, it  follows  with  great  probability  that  sex  in 
such  cases  is  determined  before  or  at  the  time  of 
fertilization. 

This  conclusion  finds  strong  support  from  the  condi- 


238  HEREDITY  AND  SEX 

tions  that  have  been  made  out  in  the  armadillo. 
Jehring  first  reported  that  all  the  young  of  a  single 
litter  are  of  the  same  sex  (Fig.  111).  The  statement 
has  been  verified  by  Newman  and  by  Patterson  on  a 
large  scale.  In  addition  they  have  found,  first,  that 
only  one  egg  leaves  the  ovary  at  each  gestative  period ; 
and  second,  that  from  the  egg  four  embryos  are  pro- 


FIG.  111.  —  Nine-banded  Armadillo.     Four  identical  twins  with  a 
common  placenta.     (After  Newman  and  Patterson.) 

duced  (Fig.  112).  The  material  out  of  which  they 
develop  separates  from  the  rest  of  the  embryonic 
tissue  at  a  very  early  stage.  The  four  embryos  are 
identical  quadruplets  in  the  sense  that  they  are  more 
like  each  other  than  like  the  embryos  of  any  other 
litter,  or  even  more  like  each  other  than  they  are  to 
their  own  mother. 

The  second  source  of  evidence  concerning  sex-deter- 


SPECIAL  CASES  OF  SEX-INHERITANCE         239 


ruination  in  man  is  found  in  the  heredity  of  sex-linked 
characters. 

The  following  cases  may  well  serve  to  illustrate 
some  of  the  better  ascertained  characters.  ~  The  tables 
are  taken  from  Davenport's  book  on  "Heredity  in 
Relation  to  Eugenics."  The  squares  indicate  males, 
affected  males  are  black  squares^- the  heavy  circles  indi- 
cate females,  that  are  supposed  to  carry  the  factors,  but 


FIG.  112.  —  Nine  banded  Armadillo.  Embryonic  blastocyst  that  has 
four  embryos  on  it,  two  of  which  are  seen  in  figure.  (After  Newman  and 
Patterson.) 

such  females  do  not  exhibit  the  character  themselves. 
Solid  black  circles  stand  for  affected  females. 

Haemophilia  appears  in  affected  stocks  almost  ex- 
clusively in  males  (Fig.  113).  Such  males,  mating 
with  normal  females,  give  only  normal  offspring,  but 
the  daughters  of  such  unions  if  they  marry  normal 
males  will  transmit  the  disease  to  half  of  their  sons. 
Affected  females  can  arise  only  when  a  haemophilious 
male  marries  a  female  carrying  haemophilia.  If  we 


240 


HEREDITY  AND  SEX 


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SPECIAL  CASES 


FIG.  114.  —  Diagram  to  indicate  heredity  of  color  blindness  through 
male.  A  color-blind  male  (here  black)  transmits  his  defect  to  his  grandsons 
only. 


XX 


50CX 


9 


cr       cr 


xx 


FIG.  115.  —  Diagram  to  indicate  heredity  of  color  blindness  through 
female.  A  color-blind  female  transmits  color  blindness  to  all  of  her  sons, 
to  half  of  her  granddaughters  and  to  half  of  her  grandsons. 


242 


HEREDITY  AND   SEX 


substitute  white  eyes  for  haemophilia,  the  scheme 
already  given  for  white  versus  red  eyes  in  flies  applies 
to  this  case.  If,  for  instance,  the  mother  with  normal 
eyes  has  two  X  chromosomes  (Fig.  114),  and  the  fac- 
tor for  haemophilia  is  carried  by  the  single  X  in  the 
male  (black  X  of  diagram),  the  daughter  will  have 
one  affected  X  (and  in  consequence  will  transmit  the 
factor),  but  also  one  normal  X  which  gives  normal 


FIG.  116.  —  Pedigree  of  Ichthyosis  from  Bramwell.     (After  Davenport.) 

vision.  The  sons  will  all  be  normal,  since  they 
get  the  X  chromosomes  from  their  mother.  In  the 
next  generation,  as  shown  in  the  diagram  (third  line), 
four  classes  arise,  normal  females,  hybrid  females,  normal 
males,  and  haemophilious  males.  Color  blindness  fol- 
lows the  same  scheme,  as  the  above  diagrams  illustrate 
(Figs.  114  and  115).  In  the  first  diagram  tfoe  color- 
blind male  is  represented  by  a  black  eye ;  the  normal 
female  by  an  eye  without  color.  The  offspring  from 


SPECIAL  CASES  OF  SEX-INHERITANCE        243 


0 

—6 

Q 

I-D 

N 

>ni  Herringham.  (After  Davenport.) 

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r-0 

D 
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D                           D 

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244  HEREDITY  AND  SEX 

two  such  individuals  are  normal,  but  the  color  blindness 
reappears  in  one-fourth  of  the  grandchildren,  and  in 
these  only  in  the  males.  The  reverse  mating  is  shown 
in  the  next  diagram  in  which  the  female  is  color-blind. 
She  will  have  color-blind  sons  and  normal  daughters 
(criss-cross  inheritance),  and  all  four  kinds  of  grand- 
children. 

Other  cases  in  man  that  are  said  to  show  sex-linked 
inheritance  are  atrophy  of  the  optic  nerve,  multiple 


T 


6     •  <^ 


ipOBiQ 

.  •  ... 


T 


...•....* 


FIG.  118.  —  Pedigree  of  night  blindness  in  a  negro  family,  from  Bordley. 
(After  Davenport.) 

**IA 

sclerosis,  myopia,  ichthyosis  (Fig.  116),  muscular 
atrophy  (Fig.  117).  Night  blindness  is  described  in 
certain  cases  as  sex-linked ;  in  other  cases,  however,  a 
disease  by  the  same  name  appears  to  be  a  simple  domi- 
nant and  not  sex-linked  (Fig.  118). 

All  these  cases  of  sex-linked  inheritance  in  man 
are  explained  by  the  assumption  that  the  factor  that 
produces  these  characters  is  carried  by  the  sex  chromo- 
some, which  is  duplex  (XX)  in  the  female  and  simplex 
(X)  in  the  male.  A  simpler  assumption  has  not  yet 
been  found.  If  one  is  fastidious  and  objects  to  the 


SPECIAL   CASES  OF  SEX-INHERITANCE         245 

statement  of  factors  being  carried  by  chromosomes,  he 
has  only  to  say,  that  if  the  factors  for  the  characters 
follow  the  known  distribution  of  the  sex  chromosome, 
the  results  can  be  accounted  for. 

The  culmination  of  the  evidence  of  sex-determina- 
tion in  man  is  found  in  a  study  of  the  cell  structure 
of  the  human  race  itself.  Strange  as  it  may  seem,  we 
have  been  longer  in  doubt  concerning  the  number 
of  chromosomes  in  man  than  hi  any  other  animal  as 
extensively  studied.  Four  conditions  are  responsible  : 

(1)  The  large  number  of  chromosomes  present  in  man. 

(2)  The  clumping  or  sticking  together  of  the  chromo- 
somes.    (3)  The  difficulty  of  obtaining  fresh  material. 
(4)  The  possibility  that  the  negro  race  has  half  as  many 
chromosomes  as  the  white  race. 

Two  years  ago  Guyer  announced  the  discovery  that 
in  all  probability  there  exist  in  man  two  unpaired 
chromosomes  in  the  male  (Fig.  119)  that  behave  in  all 
respects  like  that  in  the  typical  cases  of  the  sort  in 
insects,  where,  as  we  have  seen,  there  are  two  classes 
of  spermatozoa,  differing  by  the  addition  of  one  more 
chromosome  in  one  class.  These  produce  females ;  the 
lacking  class  produces  male's.  But  Guyer's  evidence 
was  not  convincing.  He  found  in  all  12  chromosomes 
in  one  class  of  sperm  and  10  hi  the  other.  Mont- 
gomery has  also  studied  the  same  problem,  but  his 
account,  while  confirming  the  number,  is  in  disagree- 
ment in  regard  to  the  accessory. 

Jordan  has  gone  over  a  number  of  other  mammals, 
in  some  of  which  he  thinks  that  he  has  found  indica- 
tions at  least  of  two  classes  of  sperm. 

Still  more  recently  another  investigator,  von  Wini- 


246  HEREDITY  AND   SEX 

warier,  has  attacked  the  problem  (Fig.  120).  His 
material  and  his  methods  appear  to  have  been  superior 
to  those  of  his  predecessors.  His  results,  while  stated 
with  caution  and  reserve,  seem  to  put  the  whole 
question  on  a  safer  basis. 

His   main   results   are   illustrated   in    the    diagram 


>fr    $$!. 
I  j»   4* 


FIG.  119.  —  Human  spermatogenesis  according  to  Guyer.      The  sex 
chromosomes  are  seen  in  6-9. 

(Fig.  120).  In  the  male  he  finds  47  chromosomes. 
Of  these  46  unite  at  reduction  to  give  23  double 
chromosomes  —  one  remains  without  a  mate.  At  the 
first  reduction  division  the  pairs  separate,  23  going 
to  each  pole,  the  unpaired  chromosome  into  one  cell 
only. 


SPECIAL   CASES  OF  SEX-INHERITANCE         247 

At  the  next  division  all  the  chromosomes  in  the  23 
group  divide,  likewise  all  in  the  24  group  divide. 
There  are  produced  two  spermatozoa  containing  24 


• 

<#»' 


FIG.  120.  —  Human  spermatogenesis  according  to  von  Winiwarter.  a, 
spermatogonial  cell  with  duplex  number;  b,  synapsis ;  c,  d,  e,f,  first  spermato- 
cytes  with  haploid  number  of  chromosomes ;  g,  first  spermatocyte  division, 
sex  chromosomes  (below)  in  advance  of  others ;  h,  two  polar  plates  of  later 
stage ;  i,  first  division  completed ;  /,  second  spermatocyte  with  23  chromo- 
somes ;  k,  second  spermatocyte  with  24  chromosomes ;  /,  second  spermato- 
cyte division ;  m,  two  polar  plates  of  later  stage. 


248  HEREDITY  AND  SEX 

chromosomes,  and  two  containing  23  chromosomes  ; 
all  four  sperms  having  come  from  the  same  spermato- 
gonial  cell  (Fig.  121). 

In  the  female  von  Winiwarter  had  difficulty  in  deter- 
mining   the    number    of    chromosomes    present.     His 


.  v,  ,-    determination    in   /Han 


~{~<l 

,;•>, 

^7  ** 


„  / 


N<">IW|H»»»»l  '' 


• 

FIG.  121.  —  Diagram  of  human  spermatogenesis.  A,  spermatogonial 
cell  with  47  chromosomes ;  B,  first  spermatocyte  with  reduced  haploid  number 
and  sex  chromosome  (open  circle)  ;  C,  first  division ;  D,  two  resulting  cells 
=  second  spermatocytes ;  E,  division  of  second  spermatocytes ;  F,  four 
resulting  spermatozoa,  two  female-producing  (above),  two  male-produc- 
ing (below). 

best  counts  gave  48  chromosomes  for  the  full  or  duplex 
number.  These  observations  fit  in  with  the  results 
from  the  male. 

If  these  observations  are  confirmed,  they  show  that 
in  man,  as  in  so  many  other  animals,  an  internal 
mechanism  exists  by  which  sex  is  determined.  It  is 
futile  then  to  search  for  environmental  changes  that 


SPECIAL  CASE&  OF  SEX-INHERITANCE         249 

might  determine  sex.  At  best  the  environment  may 
slightly  disturb  the  regular  working  out  of  the  two 
possible  combinations  that  give  male  or  female.  Such 
disturbances  may  affect  the  sex  ratio  but  have  nothing 
to  do  with  sex-determination. 


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ADDENDA  279 

ische  Erneurung  des  Kernapparates  ohne  Zellverschmelzung 

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larius,  Euchistus  servus,  and  the  Hybrids  of  the  FI  and  the  Fz 

Generations.     Archiv  f.  Zellf.,  XII. 
GROSVENOR,  G.  H.,  AND  GEOFFREY  SMITH,  1913.    The  Life  Cycle 

of  Moina  rectirostris.     Q.  J.  M.  S.,  LVIII. 
HYDE,  R.  R.,  1914.     Fertility  and  Sterility  in  Drosophila  ampelo- 

phila.     Jour.  Exp.  Zool.,  XVII. 
ISSAKOWITSCH,  A.,  1908.     Es  besteht  eine  zyklische  Fortpflanzung 

bei  den  Cladoceren,  aber  nicht  im  Sinne  Weismanns.     Biol. 

CentralbL,  XXVIII. 
MARCHAL,   E.   AND   E.,    1907.     Aposporie  et  sexualite   chez   les 

mousses.     Bull.  Ac.  R.  Belgique,  Classe  Sciences,  1907. 
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Exp.  Zool,  XVII. 

MITCHELL.  ('.  W.,  1913.     Sex-determination  in  Asplanchna  am- 
phora.    Jour.  Exp.  Zool.,  XV. 

MORGAN,  T.  H.,  1914.     Two  Sex-linked  Lethal  Factors  in  Dro- 
sophila and  their  Influence  on  the  Sex-ratio.     Jour.  Exp.  Zool., 

XVII. 

MORGAN,  T.  H.,   1914.     Mosaics  and  Gynandromorphs  in  Dro- 
sophila.    Proc.  Soc.  Exp.  Biol.  and  Med.,  1914. 
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Jahrb.  Abt.  Syst.,  XII. 
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XXX. 
POTTS,  F.  A.,  1906.     The  Modification  of  the  Sexual  Characters  of 

the  Hermit  Crab  caused  by  the  Parasite  Peltogaster.     Q.  J. 

M.  S.,  L. 
RORIG,  A.,   1899.     Welche  Beziehungen   bestehen   zwischen  den 

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INDEX 


Abraxas,  128 
Achates,  151 
Achia,  106 

Addison's  disease,  147 
Adkins,  217-218 
Adrenal,  147 
Agenor,  151 
Allen,  113 
Amphibia,  145 
Amphipoda,  117 
Andrews,  117 
Angiostomum,  170 
Antlers,  110,  133 
Ants,  117 
Argentine,  227 
Argonauta,  26 
Aristotle,  35 
Armadillo,  238 
Ascaris,  20,  21, -19 
Ascidian,  217 

Baltzer,  55,  58,  61 

Bancroft,  194 

Barnacle,  155 

Bateson,  72,  75,  99,  100,  125 

Baur,  E.,  99 

Beans,  123 

Bee,  174,  175,  176,  220 

Bees,  32 

Beetles,  106 

Bell,  232 

Belt,  102 

Bird  of  paradise,  king,  109 

six-shafted,  109 

superb,  109 
Black,  96-97 
Blakeslee,  171 
Bobolink,  27 
Boring,  51 
Boveri,    51,    55,    58,    162,    165, 

171 

Bresca,  145 
Bridges,  223,  224 


170, 


Bruce,  212 
Bryonia,  171-172 
Biitschli,  8 

Calkins,  8,  198,  206,  209,  210 

Callosamia,  116 

Capons,  142,  143 

Cardamine,  215-216 

Castle,  195 

Ceylon,  125,  127 

Checker  diagram,  78 

Chemotaxis,  117 

Chidester,  117 

Cicada,  106 

Ciona,  217-218 

Clipped  wings,  119 

Colias,  129-130,  150 

Collins,  202 

Color-blind,  241 

Color  blindness,  242 

Conger  eel,  2 

Corpus  luteum,  147 

Correns,  74,  79,  99,  171,  172.  215.  216 

Crab,  155 

Cretinism,  146 

Cricket,  150 

Criodrilus,  168 

Cuenot,  232,  233 

Cunningham,  121 

Daphnians,  182-185,  189 

Darwin,  C.,  73-74,  101,  103,  104,  107, 

112-114,   120,   125,   142,   194,   197, 

200-202 

Davenport,  C.,  72,  143,  239 
Deer,  110,  133,  134 
Delage,  193 
Dinophilus,  234 
Diplogaster,  225 
Doncaster,  176 

Dorsets,  134,  135,  136,  137,  138 
Drelincourt,  232 
Drone,  175 


281 


282 


INDEX 


Drosophila,  63-68,  96,  117,  130 
Busing,  233 

East,  99,  202,  204,  211 
Edwards,  51 
Egret,  111 
Eland,  136 
Elaphomyia,  106 
Elephant,  110 
Emerson,  99 
Eosirt  eye,  130,  154,  155 
Eupaguras,  158 
Euschistus,  151 

Fabre,  220,  221 

Fielde,  117 

Firefly,  28,  30,  31 

Fishes,  32 

Florisuga,  102 

Foot,  151 

Forel,  117 

Frog,  145,  147,  228 

Frolowa,  51 

Fruit  fly,  117,  195,  196,  221 

Fundulus,  32 

Gall,  179 
Galton,  236 
Game,  144,  212 
Geddes,  232 
Gentry,  232 
Germ-cells,  23 
Gerould,  130,  150,  151 
Giard,  155 
Gigantism,  146 
Goldschmidt,  124 
Goodale,  72,  142 
Gosse,  103     : 
Growth,  3 
Gudernatsch,  147 
Guinea  hen,  225 
Gulick,  51 

Guyer,  225-226,  245 
Gynandromorphism,  161 
Gypsy  moths,  117 

Habrocestum,  107 
Haemophilia,  239,  240,  242 
Hectocotylized  arm,  26 
Henking,  50 
Herbst,  55,  61,  62 
Herdwicks,  134-135 


Hermaphroditism,  161 
Hertwig,  R.,  9,  228,  234 
Holmes,  117 
Hormones,  146 
Horns,  133-138 
Hudson,  114,  115 
Humming-birds,  103,  108 
Hydatina,  2,  185 
Hyde,  196,  199,  215 

Ichthyosis,  242 
Identical  twins,  236-239 
Inachus,  155 
Ipomcea,  197 
Italian,  227 

Jacobson,  151 

Janda,  168 

Janssens,  94 

Jehring,  238 

Jennings,  9,  12,  206-208 

Johannsen,  122-125 

Jordan,  H.  E.,  245 

Keeble,  212 
Kellogg,  117,  232 
King,  229,  234 
Kopec,  149 
Kruger,  225 
Kuschakewitsch,  228 

Lamarckian  school,  17 

Landois,  232 

Langshan,  69-71 

Laomedon,  151 

Lethal  factor,  221-223 

Linkage,  93 

Lion,  27 

Lister,  34 

Loeb,  J.,  62,  190,  191,  192,  193 

Lutz,  118 

Lychnis,  172-173 

Lygseus,  44 

Lymantria,  148 

McClung,  50 

Maevia,  108 

Mallard,  28,  142 

Malsen,  von,  234,  235,  236 

Mammals,  159 

Mammary  glands,  140 

Man,  34,  229,  236-249 


INDEX 


283 


Marchals,  171 

Mast,  30 

Maupas,  5,  8,  187,  198,  234 

Mayer,  116 

de  Meijere,  151 

Meisenheimer,  145,  148-149 

Mendel,  84,  73-75,  80,  84 

Menge,  34 

Merino,  134,  135 

Miastor,  21,  174 

Mi<-<>.  233 

Mimicry,  127-130 

Miniature  wings,  66-67 

Mirabilis,  79-80 

Moenkhaus,  196 

Montgomery,  34,  50,  115,  117,  245 

Mosquito,  51 

Mosses,  171 

Mulsow,  51 

Myopia,  242 

Xematode,  224-226 
Nereis,  36 

Neuroterus,  176-177 
Nswmann,  238 
Night  blindness,  242 
Non-disjunction,  223-224 
Xussbaum,  16,  145 

Ocneria,  148 
Octopus,  25 
Oncopeltus,  46,  84 
Optic  nerve  atrophy,  244 
Oudemans,  148 
Ovariotomy,  135 
Owl,  111 

Papanicolau,  183-185 

Papilio,  125-129,  151 

Paramoecium,  5,  6,  12,  206-211 

Parathyroid,  146 

Parthenogenesis,  161 

Patterson,  239 

Paulmier,  50 

Pea,  edible,  75-78,  85-88 

Pearl,  R.,  72,  212-213,  227 

Pearse,  117 

Peckham,  115-116,  120 

Pellew,  212 

Peltogaster.  158 

Petrunkewitsch,  117 

Phalarope,  112 


Pheasants,  225 

Phidippus,  34 

Photinus,  28 

Phylloxerans,  52,  54,  178,  179,    180, 

181,  189 
Pigeons,  32 
Pituitary  body,  146 
Plutei,  60 

Plymouth  rock,  69-71,  212 
Polar  bodies,  37 
Polytmus,  103 
Porter,  117^ 
Porthetria,  117,  148 
Primula,  201,  202 
Promethea,  116 
Protenor,  40 
Punnett,  127,  128,  138,  233 

Rawls,  221 
Rat,  140,  233 
Reduplication,  100 
Reindeer,  136 
Rhabditis,  169,  224,  226 
Riley,  232 
Ritzema-Bos,  195 
Rotifers,  185-189 
Rudimentary  wing,  214,  215 
Russo,  234 

Sacculina,  155 

Sagitta,  21,  22 

Schenk,  233 

Schleip,  170,  171 

Schultze,  233 

Sclerosis,  242 

Seabright,  143-144 

Sea  cow,  27 

Sea-lion,  Steller's,  110 

Sea-urchin,  56-62 

Segregation,  81,  100 

Sex,  83,  84 

Sex  chromosome,  50,  80,  83,  84 

Sex  determination,  84 

Sex-limited,  83 

Sex-linked,  81,  83,  84,  132 

Sheep,  134-138 

Shull,  A.  F.,  187,  197,  205 

Shull,  G.  H.,  173,  202,  204,  211 

Shuster,  145 

Siamese  twins,  236 

Silkworm,  117,  165 

Sinety,  50 


284 


INDEX 


Skeleton,  rat,  140 
Smith,  G.,  145,  155 
Soule,  116 
Sparrow,  2 
Spermatophores,  25 
Sphaerechinus,  59-60 
Spiders,  34,  107,  115,  117 
Squid,  24 
Stag,  133 
Steinach,  140 
Stephanosphaera,  5 
Stevens,  51 
Strobell,  151 

Strongylocentrotus,  59,  60,  62 
Sturtevant,  72,  98,  117,  118 
Stylonichia,  2 
Suffolks,  136-138 
Synapsis,  93 

Tadpoles,  147 
Tanager,  scarlet,  27 
Thomson,  232 
Thymus,  146-147 
Thyroid,  146-147 
Toad,  229 
Tower,  117 
Toyama,  165 
Treat,  232 


Triton,  145 
Trow,  99 
Tschermak,  74 

Vermilion  eye,  119 
Vestigial  wing,  96-97 
Vigor,  120 
Vincent,  146 
de  Vries,  74,  125 

Wallace,  102,  113-114,  120,  125,  127 

Wasp,  220 

Weismann,  16,  17,  40,  194,  195 

Wheeler,  117 

White  eye,  62-65,  81,  82,  88-92,  118, 

119,  221-223 

Whitney,  185,  187,  197,  205 
Wilder,  237 
Wilson,  51 
Winiwarter,  245-248 
Wood,  136 
Woodruff,  8,  198 

X-chromosome,  51,  82,  84,  242 

Y-chromosome,  51,  84 

Yellow  body  color,  67,  88-92,  119 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPEDJiELOW 

AN     INITIAL     FlNEToF     25     CENTS 

OVERDUE. 


OCT   0    1932 
OCT  23  1932 
JAN  31 1933 


COT  21 1933 

NOV   27  1933 

(\    V     <wO  I 

DF3    4  1933 


OCT  2  4  1935 


OCT  1 4  1936 


MOV 

MAR  2 


MAR      9  1953 

SEP 
SEP    9  1968  IS 


LD  21- 


300295 


QH43I 


'9/4 

THE  UNIVERSITY  OF  CALIFORNIA  LIBRAR* 


